Mix the Dry Ingredients: Start by combining the cocoa, cornstarch, sugar, and salt in a medium-sized saucepan. This ensures that all the dry ingredients are evenly distributed before adding the wet ingredients.

Add the Wet Ingredients: Gradually add the beaten egg yolks and milk to the saucepan, stirring continuously to avoid lumps. The key is to add the liquids slowly while mixing to create a smooth, lump-free mixture.

Cook the Filling: Place the saucepan over medium-high heat. Cook the mixture, stirring constantly, until it thickens to a pudding-like consistency. This step is crucial as it ensures the pie filling is thick and creamy. Once thickened, remove the saucepan from the heat and stir in the vanilla extract.

Pour into Pie Shell: Pour the thickened chocolate mixture into a pre-baked pie shell. The pre-baking ensures that the crust remains crisp and doesn’t become soggy once the filling is added.

Chill the Pie: Place the pie in the refrigerator to chill. This allows the filling to set completely, making it easier to slice and serve.