Make the Crust: Preheat the oven to 350°F (175°C). In a bowl, combine crushed Oreo cookies and melted butter. Press the mixture into the bottom and up the sides of a 9-inch pie plate.
Prepare the Filling: In a saucepan, whisk together sugar, cornstarch, and salt. Gradually whisk in milk. Cook over medium heat, stirring constantly until the mixture thickens and begins to bubble.
Add Egg Yolks: In a separate bowl, whisk the egg yolks. Slowly add a few spoonfuls of the hot mixture to the yolks to temper them, then whisk the yolk mixture back into the saucepan. Cook for another minute until thick.
Incorporate Chocolate: Remove from heat and stir in the chopped bittersweet and unsweetened chocolate, butter, and vanilla extract until smooth.
Pour the filling into the cooled crust and smooth the top. Cover with plastic wrap and refrigerate for at least 4 hours or until set.
Make the Whipped Topping: In a mixing bowl, whip the heavy cream with powdered sugar until soft peaks form. Spread the whipped cream over the chilled filling.
Garnish and Serve: Top with chocolate shavings or curls. Slice and serve chilled.