Step 1

Cook and stir milk, vanilla extract, and butter in a saucepan over medium heat until simmering. Remove mixture from heat before it comes to a boil.

Step 2

Whisk eggs, sugar, and cornstarch together in a bowl until sugar dissolves.

Step 3

Set saucepan back over low heat. Pour in egg mixture slowly, whisking constantly, until custard thickens enough to coat the bottom of a spoon, 5 to 10 minutes.