Whisk Dry First: In a medium saucepan, combine sugar, cornstarch, and salt. Whisk them well to avoid lumps.
Mix Wet: In a separate bowl, whisk your egg yolks, then blend in the milk and heavy cream.
Temper Eggs: Slowly pour about half of the milk mix into the egg yolks, whisking constantly. This warms the eggs gently, so they don't scramble.
Cook & Thicken: Pour the tempered egg mix back into the saucepan with the rest of the milk. Cook over medium heat, whisking non-stop, until it thickens and gently boils. Keep whisking for 1-2 minutes more.
Finish It Up: Remove from heat. Stir in butter until melted, then add vanilla extract.
Chill Time: Transfer custard to a bowl. Cover the surface directly with plastic wrap to prevent a skin. Let it cool completely, then chill in the fridge for at least 1 hour (or until well-chilled).