Pastry Cream for Chestnut Chocolate Layer Cake

In a medium saucepan, bring the milk to a boil. Fill a large bowl with ice and water; set aside.

In a medium bowl, combine cornstarch, flour, sugar, salt, egg, and egg yolk; whisk until well combined. Pour one-quarter of the boiling milk into egg mixture, whisking until combined. Whisk mixture back into remaining milk; place over medium-low heat. Whisk until mixture begins to bubble and has thickened, 4 to 5 minutes.

Remove from heat; whisk in butter and vanilla until butter is completely melted.

Transfer mixture to a medium bowl; place bowl in the ice bath. Let stand until cold, stirring occasionally, about 10 minutes. Cover with plastic wrap, and store in the refrigerator until ready to use.