In a medium bowl, whisk the egg yolks with ½ cup milk, sugar, cornstarch, and salt.
In a double boiler, heat 1½ cups milk until steaming.
In a steady stream, whisk the hot milk into the egg yolk mixture.
Return the mixture to the double boiler and stir continuously over medium heat. The custard will slowly warm up, then thicken all of a sudden. Remove from the heat.
Stir in the butter and vanilla extract.
Let cool.
Recipe by Kiku Corner at https://www.kikucorner.com/2015/01/03/easy-creamy-vanilla-custard/