In a medium saucepan, mix cornstarch, sugar, and salt. In a separate bowl, whisk together milk and egg thoroughly and then gradually stir into your saucepan.
Over medium heat, cook until mixture comes to a boil, stirring constantly.
Let the mixture heat up slowly. If you decide to turn up the heat to get this to boil faster, you will end up with a scrambled egg mixture!
Boil for 1 minute while still stirring constantly. Remove from heat. Stir in butter and vanilla. We usually serve warm but you can also serve chilled.