If using a store-bought crust, follow package instructions to pre-bake it.

In a medium bowl, whisk together the granulated sugar, unsweetened cocoa powder, cornstarch, and salt.

In a separate bowl, whisk the egg yolks and milk together until well blended.

In a saucepan, combine the dry mixture with the milk and egg mixture over medium heat.

Stir constantly until the mixture thickens and starts to bubble, which usually takes about 5-10 minutes.

Once thickened, remove from heat and stir in the unsalted butter and vanilla extract.

Carefully pour the chocolate filling into the prepared pie crust.

Cover the pie with plastic wrap and refrigerate for at least 4 hours, or until set.

In a mixing bowl, whip the heavy cream with powdered sugar until soft peaks form.