Add 24 Oreo cookies to the bowl of a food processor. Cover and pulse until they become fine crumbs. Add 5 tablespoons butter and pulse again until the mixture resembles wet sand.
Pour the Oreo crumbs into a 9-inch deep dish pie plate. Press them evenly along the bottom, then up the sides to form a crust. Bake in the preheated oven for 10 minutes, or until the crust looks dry. Allow to cool completely on a wire rack, about 1 hour.
Pudding
In a medium saucepan, whisk together 1 cup granulated sugar, 1/4 cup unsweetened cocoa powder, 1/4 cup cornstarch, and 1/2 teaspoon salt. Whisk in 1 cup heavy whipping cream, then 3 large egg yolks and finally 2 cups whole milk, mixing well after each addition.
Place the saucepan over medium heat and cook, stirring constantly, until the mixture starts to bubble and thickens enough to coat the back of a spoon.
Immediately remove from the heat and stir in 1/2 cup semi-sweet chocolate chips, 2 tablespoons unsalted butter, and 1 teaspoon vanilla extract until completely melted.
Pour the pudding into the cooled crust, then press plastic wrap directly onto the surface of the pudding to prevent a skin from forming. Let it cool at room temperature for 30 minutes, then refrigerate for at least 4 hours, or until fully chilled and set.
Topping
When you're ready to serve, beat 1 cup heavy whipping cream, 3 tablespoons powdered sugar, and 1/4 teaspoon vanilla extract in a large mixing bowl until stiff peaks form.
Spread the whipped cream over the cooled pie, then sprinkle with dark chocolate shavings. Slice and serve.
Notes
If you don’t have a food processor, crush the Oreos in a sealed gallon-size bag with a rolling pin.
For a less rich filling, use 3 cups of whole milk and skip the heavy cream in the pudding.
For the best look, add the whipped cream shortly before serving. Leftovers will keep covered in the fridge for 3 to 4 days, but the whipped cream may soften.
This pie can be made one day ahead. Chill the filled pie without the topping, then add the whipped cream and chocolate shavings before serving.