Just one look at this cake leaves me grinning. Everything from the Wonka-esque candy topping to the rainbow of colorful cake layers makes me giddy. And to really gild the lily, the center of the cake is filled with sprinkles and even more candy, releasing an avalanche of joy when you cut into it.
Maybe you've seen cakes like this on Pinterest, your favorite baker's Instagram feed, or even IRL at your best friend's house and thought to yourself, "That cake looks so fun and complicated; I could never make that." I'm here to encourage you otherwise! You can totally make this knock-out, show-stopping surprise-inside rainbow layer cake from scratch.
Yes, it will take some time and some planning, but we've got the step-by-step instructions to make this great-looking (and great-tasting) cake at home.
Ultimate Rainbow Surprise Cake: Watch the Video
Why You Should Make Rainbow Surprise Cake
This rainbow cake is the most fun, over-the-top celebration cake, perfect for a surprise birthday party. With a technique that's easier than even the simplest surprise-inside cake, a rainbow layer cake filled with colorful candy will delight your birthday party guests of all ages. Fluffy vanilla cake makes up the colorful layers, while sweet, rich buttercream frosting make this surprise cake taste even better than it looks. It is, dare I say, even more fun than Funfetti.
This cake is going to take more time than your average birthday cake, but if you break the preparation up over two or three days, it makes the process smoother, less rushed, and easier to accomplish. Up first? Coloring and baking the rainbow cake layers.
The Best Cake for Rainbow Cake Is White Vanilla Cake
Snow-white cake batter takes readily to a rainbow of hues — no fighting yellow cake's buttery color. This fluffy white cake calls for cake flour, making it less susceptible to overmixing (a legit concern when you must divide and color the batter six different ways).
Otherwise, there's nothing different about the batter despite the coloring that will happen later. Butter and sugar are creamed together with a paddle attachment on a stand mixer, then egg yolks are added before the dry ingredients are alternated with the milk. You'll want to stop and scrape down the sides of your mixing bowl regularly, as this is a large volume of cake and the creamed butter and sugar often hide in the bottom of the bowl. Now you've got your basic batter and it is time to make those rainbow layers.
Need a refresher? How To Make a Cake from Scratch
How to Make One Cake Batter and Color It 6 Ways
We now come to our first hurdle of baking this cake: You've got six layers to bake and only two cake pans (or you've got three or more cake pans, but not an oven large enough to accommodate them all), but that is more than fine! Cake batter is more resilient than we give it credit for.
We're going to mix up two layers at a time. When you pair the colors correctly, you won't have to wash the bowl between every single batter.
Now, you are going to have to wait for the first two layers to bake and cool enough to remove the cake from the pans, clean your baking pans, and repeat dividing the batter, baking, and cooling, but the cakes only bake for 20 minutes and can be removed from the pan before they are completely cool. Cooling for 30 minutes will make for cakes that are study enough for removal without damage.
Here's the order to follow and how to do it.
- Red batter then orange batter.
- Yellow batter then green batter.
- Blue batter then purple batter.
- First, coat two (9-inch) cake pans with cooking spray and a round of parchment paper.
- Then weigh out 12 1/2 ounces of batter (this is roughly 2 cups) into a clean bowl. Add a teaspoon of red food coloring and mix gently to combine.
- Pour the red batter into one of the prepared cake pans, scraping as much batter from the bowl as possible.
- Now, weigh out another 12 1/2 ounces of batter (again, close to 2 cups of batter) and this time add a teaspoon of orange food coloring. Put these two cakes in the oven to bake. Cover the mixing bowl of batter with plastic wrap and clean and dry the bowl you used for coloring.
Pro tip: Wrapping the cakes in plastic wrap while they are still slightly warm will make for softer edges, tops, and bottoms on the cooled cakes. Make sure to leave the parchment paper round on the bottom of the cakes for easier maneuvering later.
Mistakes to Avoid When Building a Rainbow Cake
- Don't rush: Bake the cakes one day and assemble the next. Plan to refrigerate the cake for either a few hours or overnight before serving, so make space for it ahead of time.
- Don't shortcut the cavity building: Use a circular cutter for removing the center from two cake layers, instead of building the cake and then cutting out a hole with a paring knife. Your cake will have a more polished, professional look from this one maneuver.
- Don't just throw candy in there: Line the cavity with frosting so that some of the sprinkles and candy will stick to the insides of the cavities. This will make the opening look fuller as the candy falls out. Use coated candy and jimmy sprinkles for the longest-lasting candy filling. Chocolate and gummy candies sweat inside the cake.
- Don't cut the cake right away: The cake will be sturdier after the frosting has set, making for a cleaner-slicing, easier-to-serve cake.
The Best Frosting for Rainbow Cake Is Simple
Here is my philosophy — invest your baking time in making the beautiful rainbow layers and make a basic buttercream. You can use your favorite recipe, but will need to double the amounts for filling and covering this cake.
Buttercream basics: How To Make a Basic Buttercream Frosting
How Do I Actually Build a Rainbow Surprise Cake?
You've got your baked cake layers; your buttercream frosting; and about two cups of sprinkles, coated candies, and edible glitter and now you must build the cake. Don't panic! Decide in advance the order your layers will go in by stacking the wrapped cakes. And finally split your six cakes into three sets of two: bottom layers, middle layers, and top layers. The two middle layer cakes will get a hole in the middle.
Pick a cake plate and line it with parchment paper: Put a dab of frosting in the center of your cake plate. Line the cake plate with 4 strips of parchment paper to keep the cake plate clean while you frost.
Unwrap, stack, and frost the two bottom cakes: Unwrap the purple cake layer and remove its parchment paper. Place it on the center of the cake plate. Top with 3/4 cup of frosting and smooth into an even layer. Some crumbs may stick into the frosting, but don't worry — you won't see them once the cake is complete. Unwrap the blue cake layer and remove the parchment paper round. Invert the blue layer on top of the purple layer, making sure to align the two cakes as much as possible. Frost the top of the blue cake layer with another 3/4 cup frosting.
Unwrap, punch holes, and stack the two middle layers: Unwrap the green cake layer, but keep the parchment round on. Use a 3-inch-wide round cutter to cut a hole in the center of the green cake layer, then use an offset spatula to remove the small round of cake. Use the green cake's parchment paper to easily invert the green cake over the blue layer. Peel away and discard the parchment paper. Frost the green ring with 1/2 cup of frosting, keeping clear of the hole for now. Repeat unwrapping, punching a hole, and moving to the stack of cakes with the yellow layer. Remove the yellow cake's parchment paper and frost the yellow layer with 3/4 cup frosting. This time frosting the interior of the cake's hole with a thin layer of frosting.
Fill the cake with sprinkles! Pour the sprinkles and candy into the cavity in the green and yellow layers. Press the sprinkles into the sides and bottom of the cavity, being sure the center is quite full and packing in the sprinkles as necessary.
Unwrap, stack, and frost the top two layers: Unwrap the orange cake layer and remove its parchment paper. Place the orange cake layer on top of the yellow cake layer and align the cakes as much as possible. Frost the top with 3/4 cup of frosting. Unwrap the red cake layer, remove the parchment, and level the cake if needed. Invert the red layer on top of the orange layer, making sure to align the two cakes as much as possible.
Crumb coat the cake: Place half of the remaining frosting on top of the cake and use an offset spatula to spread it into a thin layer on the top and sides of the cake. Some crumbs will be picked by the frosting, but don't worry — you'll cover those with another thin layer of frosting in a bit. Refrigerate the crumb-coated cake for one hour.
Frost and decorate the cake. Top the cold cake with the remaining frosting and again spread into a thin, even layer on the top and sides of the cake. Top with any additional sprinkles, candies, and topping as desired.
Cutting and Serving Your Rainbow Cake
Refrigerate the finished cake for at least one hour or even overnight before slicing and serving. Use a long, sharp knife to slice through the finished cake, and be prepared that the tip of the knife will meet some resistance as it goes through the sprinkles. You'll also want a spoon to scoop some of the cake's candy filling onto each plate.
For the biggest wow presentation, try to cut out one or two pieces away from direct view of your guests and then turn it around to reveal the cake layers. Some sprinkles and candy may stick, so jostle the cake a bit to unleash the full cascade!
How To Make the Ultimate Rainbow Surprise Cake
What You Need
For the cake layers:
4 1/2 cups cake flour
1 1/2 tablespoons baking powder
1 teaspoon fine salt
3 cups granulated sugar
3 sticks (1 1/2 cups) unsalted butter, at room temperature
8 large egg yolks
2 cups whole milk
1 tablespoon vanilla extract
6 teaspoons gel food coloring (6 colors, 1 teaspoon each, such as Cake Craft Coloring Gel Startup Kit)
For the American buttercream (makes about 6 cups):
1 pound (2 cups) unsalted butter, at room temperature
7 to 8 cups powdered sugar, divided
2 tablespoons heavy cream or milk
2 teaspoons vanilla extract
For the filling and topping:
About 2 cups of a variety of edible sprinkles or candy-coated chocolate, such as:
- Wilton Gold Pearlized Sprinkles Mix
- Wilton Pearilized Sugar Food Decorative, Gold, 5.25 oz
- Wilton Edible Glitter, Gold Stars,
- Gold Sixlets Candy 1LB Bag
Measuring cups and spoons
Stand mixer with a paddle attachment
Large and small mixing bowls
2 to 6 (9-inch) round cake pans
3-inch round cutter
Wire cooling rack
Long, sharp knife
Day 1: Bake the cake layers
- Prepare your cake pans: Line 2 to 3 (9-inch) round cake pans with parchment paper rounds and coat with cooking spray. Arrange a rack in the middle of the oven and heat to 350°F.
- Combine the dry ingredients: Place the cake flour, baking powder, and salt in a large bowl and whisk to combine.
- Cream the butter and sugar: Place the butter and sugar in stand mixer fitted with a paddle attachment and beat on medium speed until the mixture is lightened in color and slightly increased in volume, about 5 minutes.
- Add the egg yolks: Stop and scrape down the sides of the mixer with a rubber spatula. Return the mixer to low speed and add the egg yolks one at a time, allowing each to fully incorporate into the batter before adding the next.
- Combine the milk and vanilla: Stop and scrape down the sides of the mixer. Combine the milk and vanilla in a measuring cup.
- Alternate beating in the dry ingredients and milk mixture: Add 1/3 of the flour mixture to the mixer and combine on low speed. Stop and scrape down the mixer. Add 1/2 of the milk mixture and incorporate on low speed. Stop and scrape down the mixer. Add another 1/3 of the flour mixture to the mixer and combine on low speed. Add the remaining milk mixture and incorporate on low speed. Stop and scrape down the mixer, then add the remaining flour mixture.
- Mix for 3 minutes on medium speed: Stop the mixer and give the whole bowl another good scrape. Get down to the bottom of the bowl where butter and sugar tend to hide and will make your cakes color and bake unevenly. Return the mixer to medium speed and beat until the batter is completely smooth, about 5 minutes.
- Divide and color the batter: You'll be baking 2 or 3 cakes at a time, depending on the number of cake pans you have and what your oven will hold. You'll have about 4 pounds, 13 ounces of batter, so weigh out 12 1/2 ounces of batter (about 2 cups) into a small bowl. Add 1 teaspoon of gel food coloring and mix well with a rubber spatula. Pour the colored batter into a prepared cake pan. Repeat with another cake layer's worth of batter, a clean bowl, and another color of food coloring. If you have a third cake pan, repeat again with a third layer of cake batter. Cover the remaining cake batter with plastic wrap and set aside while baking the cakes; it's best if you only portion the batter you need at a time.
- Bake the cakes: Bake the 2 to 3 colored cakes you have portioned and dyed until the cake is set, springs back when touched, and pulls away from the sides of the pan, 15 to 20 minutes.
- Cool and wrap the cakes: Remove the cakes to a cooling rack and cool for about 30 minutes. Remove the cakes from the pans, keep the parchment paper on, and immediately wrap each still slightly-warm cake separately in plastic wrap. Set aside at room temperature.
Repeat dividing the batter, coloring it, and baking: Wash and dry your cake pans and line again with parchment rounds and cooking spray, set aside. Repeat dividing the batter, coloring it, and baking until all of the batter is used up. Cool and wrap the remaining cakes. Set aside at room temperature for up to 1 day.
Day 2: Make the frosting and build the cake
- Beat the butter: Beat the butter in the bowl of a stand mixer fitted with a paddle attachment on medium speed until smooth.
- Gradually add in the sugar: Turn the mixer speed to low and slowly add in the 7 cups of the sugar. Once it's all added in, increase the speed to medium-low and mix until thoroughly combined.
- Add the liquid ingredients: Reduce the mixer to low, add in the cream or milk and vanilla, and mix until combined.
- Whip until creamy: Once everything is incorporated, mix on medium speed until smooth and creamy.
- Add the remaining sugar: Reduce the mixer speed to low and add in the remaining 1/2 to 1 cup of sugar until the buttercream is as firm as you want it. Turn the mixer back up to medium and whip again until fluffy.
- Designate your cake layers: Stack the still-wrapped cakes and decide what order your rainbow will be. Designate which cakes will be the middle two layers. For our purposes, purple and blue will make the foundation of the cake, green and yellow the center, and orange and red the top two layers.
- Build the foundation of the cake: Put a dab of frosting in the center of a cake or serving plate. Line the cake plate with 4 strips of parchment paper to keep the cake plate clean while you frost. Unwrap the purple cake layer and remove its parchment paper. If needed, use a long, serrated knife to level the cake. Place the leveled cake on the center of the cake plate. Top with 3/4 cup of frosting and smooth into an even layer. Some crumbs may stick into the frosting, but don't worry — you won't see them once the cake is complete. Unwrap the blue layer, remove the parchment, and level the cake. Invert the blue layer on top of the purple layer, making sure to align the two cakes as much as possible. Top with 3/4 cup of frosting and smooth into an even layer.
- Make the filled layers: Unwrap the green layer, keep the parchment on, and level if needed. Use a 3-inch-wide round cutter to cut a hole in the center of the green cake layer and use an offset spatula to remove the small round of cake. Use the green cake's parchment paper to easily invert the green cake over the blue layer and align the cakes as much as possible. Remove the parchment paper. Frost the green ring with 1/2 cup of frosting, keeping clear of the hole. Repeat unwrapping, leveling cutting a hole, and placing yellow cake on top of the green cake. Remove the parchment and frost with 3/4 cup frosting, this time also frosting the interior of the cake's hole with a thin layer of frosting.
- Fill the cake with sprinkles: Pour the sprinkles into the cavity in the green and yellow layers. Press the sprinkles in the sides and bottom of the cavity, being sure the center is quite full and packing in the sprinkles.
Make the final layers: Unwrap the orange layer, remove its parchment, and level if needed. Place the orange layer on top of the yellow cake layer and align with the other layers. Top with 3/4 cup of frosting and smooth into an even layer. Unwrap the orange layer, remove its parchment, and level if needed.
Invert the red layer on top of the orange layer and align the cakes as much as possible.
- Crumb coat the cake and chill: Place 1/2 of the remaining frosting on top of the cake and use an offset spatula to spread it into a thin layer on the top and sides of the cake. Some crumbs will be picked by the frosting, but don't worry — you'll over those with another thin layer of frosting in a bit. Refrigerate the crumb-coated cake for 1 hour. Cover the remaining frosting with plastic wrap and set aside at room temperature.
- Frost and decorate the cake: Top the chilled cake with the remaining frosting and spread into a thin, even layer on the top and sides of the cake. Top with any additional sprinkles, if desired.
Cut and serve the cake: Refrigerate the cake for at least 1 hour more or even overnight before slicing and serving. Use a long, sharp knife to slice through the finished cake, and be prepared that the tip of the knife will meet some resistance as it goes through the sprinkles.
- Make ahead: Bake the cake layers and store them at room temperature for up to 1 day.
- Storage: The assembled cake can be stored in the refrigerator for up to 5 days.