How To Make an Anatomically Correct Lego Cake

updated May 13, 2022
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(Image credit: Faith Durand)

2009_03_02-Lego.jpgSpeaking of birthday cakes, I made one last weekend. This particular cake was for a friend’s son, who was having his eighth birthday. My friend’s son is madly into LEGOs, and when he saw a building block cake on LEGO’s website, he fell in love. My friend, who is a magnificent cook but doesn’t bake, asked if I would try it. Sure! Why not?

I went looking online for LEGO cake inspiration and discovered many, many cakes — but a dearth of really practical directions.

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There are zillions of LEGO cakes out there, with varying degrees of sophisticated decoration. Want a LEGO wedding cake in progress, complete with dump trucks and cranes carrying rolls of fondant? Check. How about Indiana Jones LEGOs? Check. A 300-pound Batman LEGO cake? Check.

I was after something simpler and yet authentic. The most common recipe was at the Betty Crocker website and it called for marshmallows, canned frosting, and boxed cake mix. No way. Plus, it didn’t look very much like real LEGO bricks. My two inspirations were these cakes.

I could find very little info on the first cake, but the second cake was by a gifted amateur named Kevin. He very kindly emailed me the info on how he made his son’s cake, but it required official LEGO cake molds. I didn’t have the time to acquire those, so I was on my own. (But if I ever do this again, I’m following Kevin’s method: he painted the building blocks with melted candy to give them that shiny plastic look. Impressive!)

All I wanted was two LEGO bricks, made out of cake. But I didn’t want to just carve up a couple cakes and call it done. Proportion and dimensions are everything in LEGO engineering, and I wanted these cakes to be done right. How would I make sure the knobs were the right size, proportionally, to the height and width of the cake? How would I know if the cake was tall enough? So my husband (a real-life engineer) and I went searching for LEGO dimensions online (turns out they are easily available), did a little math, and presto – a proportional, anatomically correct LEGO cake was within reach.

So, just in case you have a LEGO geek in your life, here are step-by-step instructions on constructing an anatomically correct, perfectly proportional LEGO block.

As you’ll see near the end, the actual finishing of this cake left quite a lot to be desired: fondant is not my strong point. We joked that it was a LEGO brick that had gone through the microwave (or maybe the dog’s teeth) a few times. But it was a start, and if you’re looking to make a similar cake, maybe you can finish it off better. Here’s the basic template, with a gallery of the steps in photos above.

  1. Measurements: We were working off the DUPLO bricks, which are twice the size/dimensions of ordinary LEGO bricks. We converted the dimensions of one DUPLO brick into inches and multiplied by a factor of four. Here are the final dimensions of the cakes: • Rectangular brick: 10 inches (l) x 5 inches (w) x 3 inches (h) • Square brick: 5 inches (l) x 5 inches (w) x 3 inches (h). • Knobs: 1.5 inches diameter x 0.5 inches tall
  2. Cake pans: Fortunately these dimensions translated easily into cake pans. The knobs are the same diameter as mini-muffin tins. The cake layers were slightly smaller than 13×9 rectangular pans. The challenge was baking enough to make the cakes high enough.
  3. Recipes: In order to have enough cake, I multiplied a favorite recipe by 3.5, which gave me enough cake for 3 13×9 pans and 1 mini-muffin tin.
    Dark Chocolate Cake times 3.5
    • You also need basic buttercream or icing. Here’s one recipe: Classic Buttercream Frosting
  4. Cake Construction: Bake the cakes and cool completely – preferably overnight. Remove from the pans after about half an hour. When the cakes are cool and you’re ready to assemble, slice the rounded top off the cakes. This should leave you with about a 1-inch tall cake. Measure out a 5×10 shape and mark with toothpicks. Cut and reserve the scraps. Layer two of the 5×10 rectangles, creating a middle layer out of the scraps and spreading the buttercream between the layers. The cake should be three inches tall. Create a square brick by cutting the third 5×10 layer in half and again creating the middle layer out of scraps.
  5. Making the knobs: The mini-muffins will be just the right width on the bottom for the knobs. Hold each mini-muffin up to a ruler and get a general sense for where half an inch falls. Slice the rounded top off to make each one half an inch tall. Turn upside down and fix onto the cake with a toothpick and a smear of buttercream.
  6. Finishing: Here’s where my own plan went a little awry. I used pre-made fondant (rather nasty stuff; we call it colored glue) because I didn’t have enough time to make my own. I struggled to roll it out thin enough, and it cracked quite a lot when I put it on the cake and cake board. Next time I would probably wrap each of the knobs individually. But you could probably do better. Or you could use colored frosting to finish it instead.

And that is how to make a LEGO cake that is proportionally, anatomically correct. Have you ever made a LEGO cake? If so, share photos! And if you have any tips for others on getting that smooth, glossy LEGO finish, please do share them!

Related: Birthday Baking: Chessboard Cake with Chocolate Chess Pieces