Step 1: Mix the Base
In a heavy-bottomed saucepan, combine the cocoa, flour (or cornstarch), beaten egg yolks, sugar, and salt.
Step 2: Cook and Thicken
Gradually stir in the milk over medium-high heat. Keep whisking constantly until the mixture thickens to a pudding-like consistency—about 10 minutes. Be patient and continue stirring to keep it smooth and prevent sticking.
Step 3: Add Flavor and Butter
Remove the pan from the heat, then stir in the vanilla and butter until fully melted and incorporated.
Step 4: Fill and Chill
Pour the hot custard into the pre-baked pie shell. Refrigerate for several hours until the filling is completely set.
Step 5: Serve
Top with whipped cream if desired, or use leftover egg whites to make a meringue topping before serving.
