Mix the chocolate cookie crumbs and melted butter together. Press the mixture firmly into the bottom and up the sides of a 9-inch pie plate. Chill the crust for 15 minutes.
In a large bowl, beat the softened cream cheese and powdered sugar until smooth. Gradually beat in the 1/2 cup milk until well combined. Fold in half of the thawed whipped topping. Spread this mixture evenly over the chilled crust.
In a separate bowl, whisk the instant chocolate pudding mix with 1 1/2 cups cold milk until the mixture thickens, about 2 minutes. Spread the pudding mixture over the cream cheese layer.
Cover the pie and refrigerate for at least 4 hours, or until the filling is firm.
Before serving, top the pie with the remaining whipped topping and garnish with chocolate shavings or sprinkles.
Notes
You can use a store-bought chocolate graham cracker crust if you prefer to save time.
For a firmer, frozen pie, chill the pie for 2 hours, then freeze for 2 hours before topping.
Use high-quality chocolate cookies for the best crust flavor.