In a food processor, crush Oreo cookies into fine crumbs. Mix with melted butter until fully combined.

Press the crumb mixture firmly into the bottom and up the sides of a 9-inch pie dish. Chill in the refrigerator while preparing the filling.

In a medium saucepan, whisk together sugar, cocoa powder, cornstarch, and salt.

Add milk and egg yolks, whisking constantly over medium heat until the mixture begins to boil and thicken, about 7–10 minutes.

Remove from heat and stir in chopped chocolate, 1 tablespoon butter, and vanilla extract until smooth.

Pour the chocolate filling into the prepared crust. Press a piece of plastic wrap directly onto the surface to prevent a skin from forming.

Chill the pie in the refrigerator for at least 4 hours, or until set.

In a mixing bowl, beat heavy cream and powdered sugar until stiff peaks form. Spread whipped cream over the chilled pie.

Garnish with chocolate shavings or sprinkles if desired before serving.