Start by whisking the sugar, cornstarch, and salt in a saucepan. Slowly add the milk and cream while whisking constantly. This step ensures the cornstarch dissolves completely and prevents lumps later.
Place the saucepan over medium heat and stir continuously. After several minutes, the mixture thickens and begins to bubble gently. This indicates the cornstarch has activated and the custard is ready for eggs.
In a bowl, whisk the egg yolks. Slowly pour a small amount of hot custard into the yolks while whisking. This warms the eggs gradually so they don’t scramble.
Return the tempered egg mixture to the saucepan. Cook for another minute while stirring until the custard becomes thick and glossy. You’ll feel resistance when stirring, which is exactly what you want.
Remove the pan from heat and stir in butter and vanilla extract. The butter melts instantly, adding shine, while vanilla releases its aroma at this stage.
Pour the hot custard into the pre-baked pie crust. Smooth the top with a spatula so it chills evenly and looks neat when sliced.
Cover the surface with plastic wrap pressed directly onto the custard. Refrigerate for at least 4 hours until fully set and sliceable.