Prepare the Pie Crust: If you haven't already, bake your pie crust according to the package instructions or your favorite recipe. Allow it to cool completely.

Combine Dry Ingredients: In a medium saucepan, whisk together the sugar, cocoa powder, cornstarch, and salt until well combined.

Whisk Egg Yolks: In a separate bowl, whisk the egg yolks until they are slightly thickened and pale in color.

Add Milk and Cream: Gradually whisk in the whole milk and heavy cream into the dry mixture in the saucepan. Make sure there are no lumps.

Cook the Mixture: Place the saucepan over medium heat. Continuously whisk the mixture until it starts to thicken and bubble, about 5-7 minutes. Once thickened, remove from heat.

Incorporate Egg Yolks: Slowly add a small amount of the hot mixture to the egg yolks, whisking constantly to temper them. Then, gradually whisk the tempered egg mixture back into the saucepan.

Finish the Filling: Return the saucepan to the heat and cook for an additional 2-3 minutes, stirring constantly, until the filling is thick and glossy. Remove from heat and stir in the butter and vanilla extract until smooth.

Assemble the Pie: Pour the chocolate filling into the cooled pie crust, spreading it evenly. Cover the pie with plastic wrap or parchment paper, pressing it directly onto the surface of the filling to prevent a skin from forming.

Chill: Refrigerate the pie for at least 4 hours, or until fully set.

Serve: Before serving, top with whipped cream and garnish with chocolate shavings or a dusting of cocoa powder, as desired.