Use this recipe to make our 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.
This can be prepared up to three days in advance and stored covered in the refrigerator.