Mousseline buttercream is made by cooking sugar and water to the softball stage (238°) and then pouring this slowly into the whipped eggs while the mixer is still running. Once this is cool, the softened butter is added. This creates a much more stable frosting that is easier to work with and less prone to losing its shape once applied to your baked good.
Like other kinds of buttercream, this one can also be adapted with flavoring extracts, spices and chocolate - this recipe is a blank slate for your imagination! Add additional flavorings at the end after the butter has been fully incorporated. Reduce the mixer speed to low and start by adding a tablespoon of extract or spices. Keep tasting the frosting and adjusting until you get the flavor you want. For chocolate, start with 6 ounces finely chopped and melt it before adding.
Before trying this kind of frosting for the first time, check out our post on working with sugar. It has some helpful tips if you haven't done a lot of this type of cooking before.
Also, the sugar syrup can easily splatter while you're pouring it into the mixer bowl. Wear a long sleeved shirt and pour very slowly to avoid burning yourself. If you do get splattered, pat your skin with a clean kitchen towel first instead of running it under water. This absorbs the sugar without smearing it over your skin and burning you further.
With that charming public advisory said, let's take a look at the recipe!
Mousseline (Cooked) Buttercream
Makes enough to frost a 9" cake or approximately 24 cupcakes
6 egg whites or egg yolks (not whole egg)
pinch of salt
1 cup sugar
1/2 cup water
1/4 teaspoon cream of Tartar
2 cups butter, cut into tablespoon portions and softened
1 Tablespoon vanilla extract
In the bowl of an electric mixer, beat the yolks and salt until thickened (or until soft peaks are formed if using egg whites).
Meanwhile, combine the sugar, water, and tartar in a medium sauce pan and set over medium heat. Cook until the sugar is dissolved and the mixture reaches the softball stage (238°).
With the mixer still running and beating the eggs, pour the sugar mixture in a slow steady stream down the side of the bowl, being careful to avoid hitting the mixer's whisk. Continue beating until the egg-sugar mixture is cool and the side of the mixing bowl registers room temperature when you touch it.
Reduce speed to medium and begin beating in the butter one tablespoon at a time. Continue beating until the buttercream holds soft peaks. Add the vanilla (and any other flavorings). Chill until the buttercream is firm but spreadable.
Buttercream will keep in an air-tight container for several days.
• For some excellent buttercream recipes and variations - not to mention a ton of delicious recipes to use them on - check out Sara Kate's cookbook, The Greyston Bakery Cookbook, available for $17.16 on Amazon!