Prep oven. Position a rack in the center of the oven. Preheat oven to 350oF (180oC).
Make the crust. Add cookies to a food processor and pulse until very fine crumbs. Add melted butter and pulse to combine. Press mixture firmly into the pie plate and up the sides. Spend a few minutes making sure it’s tightly packed into the pie plate, this will help when removing slices later. I like to use the bottom of a dry measuring cup to start then finishing using my hands to really press it in.
Bake crust. Bake the crust for 8-10 minutes then set aside to cool slightly.
Combine egg yolks, sugar, cornstarch, and salt. In a large mixing bowl, whisk together the egg yolks, sugar, cornstarch, and salt until yellow and increased in volume. You can use a whisk or a mixer.
Heat milk and cocoa. In a large saucepan, whisk together the milk and cocoa powder until smooth. Heat over medium high heat until boiling.
Temper the egg yolks. Pour half of the milk mixture over the egg yolk mixture, whisking constantly. To stabilize the bowl, I like to place a kitchen towel underneath.
Heat milk/egg mixture. Pour the tempered egg yolk mixture in the bowl back into the saucepan with the remaining heated milk/cocoa mixture. Cook over low heat, whisking continuously until the mixture thickens and starts rapidly bubbling.
Strain the mixture. Set a mesh strainer over a large mixing bowl and pour the hot chocolate pastry cream mixture through a fine-mesh sieve.
Add butter, vanilla, and chocolate. Whisk in the butter, vanilla extract, and chopped chocolate until melted and combined.
Assemble and chill. Cool slightly, then pour mixture into the prepared pie crust. Cover the top of the pudding filling with plastic wrap, this helps so the filling does not form a skin. Chill in the refrigerator, to allow the filling to set up, about 6 hours, or overnight, before serving.
Make whipped cream. When ready to serve, make the whipped cream. Using an electric mixer fitted with a whisk attachment, beat the heavy cream, confectioners’ sugar, and vanilla on medium-high speed until stiff peaks form, about 5 minutes. Spread or pipe onto the chilled pie. Slice and serve.