Light yet decadent, this custard buttercream is the perfect topping to your favorite chocolate cake recipe. Made by combining pastry cream and butter, this frosting is similar in taste and texture to a whipped cream icing. Though it does require some extra work, it will be well worth it in the end! This German buttercream recipe is also less sweet than traditional American buttercream, making it a nice alternative to those who enjoy less sugary frosting.
In a medium heat-proof bowl, whisk together sugar and cornstarch until combined. Add eggs, whisking vigorously until fully incorporated.
In a medium saucepan, bring milk to simmer over medium heat stirring constantly. Ladle 1/2 cup of the hot milk into the egg mixture and whisk to combine. Repeat with two more 1/2-cup additions, then pour egg mixture back into the pan.
Cook over medium heat, whisking constantly, until the custard turns thick and lumpy, about 3-4 minutes. When custard begins to bubble, continue to whisk over heat exactly 2 minutes. Remove from heat and add vanilla and salt; stir to combine.
Spread the custard mixture into a 13 in. x 9 in. baking dish. Let stand uncovered 25 minutes in the refrigerator or 45 minutes at room temperature until custard cools to 68°F.
In a medium bowl, beat butter with an electric mixer fitted with a paddle attachment at medium speed 5 minutes or until fluffy and light in color.
Stir cooled custard base until smooth. Add custard into butter, one tablespoon at a time, mixing thoroughly until each is combined, scraping sides and bottom bowl often.
When the custard is fully incorporated, switch to the whisk attachment and whip on medium speed for 5 minutes or until the buttercream is smooth and creamy.