Makes about 560g, or about 3 cups. Making German buttercream won't take long, but bear in mind that the custard needs time to cool.
Ingredients
150ml (or ½ cup + 2 tablespoons) milk
100g (or ½ cup) granulated sugar
3 large egg yolks (55g, or a little over 3 tablespoons)
2 teaspoons cornstarch
350g (or 1½ cups + 2 teaspoons) unsalted butter, softened at room temperature
¼ teaspoon vanilla extract
table salt to taste, optional
Instructions
Combine the milk and sugar in a medium-sized saucepan. In a medium-sized bowl, whisk together the yolks, cornstarch and two tablespoons of the milk mixture until the yolk mixture looks foamy and smooth.
Place the saucepan over low heat, stirring to dissolve the sugar. Once the sugar has dissolved, crank the heat up to medium-high and scald the milk mixture. Once the first bubbles appear, remove the pan from the heat and carefully pour the hot milk mixture into the yolk mixture, whisking continuously to keep the eggs from scrambling.
Once combined, pour the mixture back into the saucepan. Heat over low heat, whisking continuously, until the first bubbles appear and the mixture thickens considerably.
Remove the pan from the heat and whisk for another minute to knock some of the heat out. Pour the custard onto a clean plate and immediately cover it with plastic wrap, pressing the plastic wrap directly onto the custard. This keeps a skin from forming. Allow the custard to cool completely.
Once the custard has cooled, beat the butter in a medium-sized bowl until smooth and fluffy and lightened in color, about 2 minutes. Add the cooled custard, one tablespoon at a time, mixing well after each addition. Once all the custard has been added, mix for another few minutes, until the buttercream looks thick, smooth and creamy. Add the vanilla (and salt to taste) and mix to incorporate.
Use immediately or store in an airtight container or a zipper bag in the fridge for up to two weeks or in the freezer for up to two months. To use buttercream that has either been refrigerated or frozen, first allow to come to room temperature then beat until smooth and spreadable again.
Cakes or cupcakes decorated with buttercream generally keep up to 3 days, stored in an airtight container in the fridge. Allow buttercream to come to room temperature before serving (for a big cake, this may take up to four hours!).