Custard-based buttercream

This German-style buttercream, based on a recipe from Cook's Illustrated, starts out with a custard made from egg yolks. It is then added to whipped butter. It is not as light as a Swiss or Italian meringue buttercream, but there's no need to make a sugar syrup as in those. If you're making a dacquoise (nutty meringue layers sandwiched with buttercream), this is a great way to use both the egg whites and yolks. The buttercream can be flavoured as desired ~ my recipe here is a coffee flavour.This buttercream remains stable for quite a while, which is great for our very warm room temperature. I have also tried making this style of buttercream with custard powder instead of eggs. Both recipes follow.

German-style Coffee Buttercream
Makes about 2 cups. Based on a Cook's Illustrated recipe.

120g egg yolks (about 4 large)

60g sugar

1 tbsp cornstarch

1/4 tsp salt

175ml full-cream milk

2 tbsp water

1-1/2 tbsp instant coffee

225g unsalted butter, softened

Whisk egg yolks, sugar, cornstarch and salt in a bowl. 

Place milk in a small pot and bring to a simmer over medium low heat. Pour half the hot milk into the yolk mixture, all the while whisking until smooth, then add this to the milk in the pot. Return the pot to the stove and continue whisking until the mixture bubbles and thickens. Transfer custard to a bowl, cover and refrigerate until set, about two hours. Before using, let custard warm up a little.

Stir water and instant coffee together; set aside.

Using electric beaters or a stand mixer, beat the butter until light and creamy. Beat in the custard in three batches until just combined. Add coffee mixture and continue to beat until mixture is light and fluffy, 3-4 minutes.

With custard powder

Place 175ml milk in a small pot and stir in 3 tablespoons custard powder, 60g sugar and a pinch of salt until well mixed. Bring to a simmer over medium low heat, whisking until  the mixture bubbles and thickens. Transfer custard to a bowl, cover and refrigerate until set, about two hours. Before using, let custard warm up a little. Using electric beaters or a stand mixer, beat 225g softened unsalted butter until light and creamy. Beat in the custard in three batches until just combined. Add flavouring to taste and continue to beat until mixture is light and fluffy, 3-4 minutes.