Step 1: Make the Cookie Crust - Place the chocolate sandwich cookies in a food processor. Pulse until they turn into fine crumbs. Pour the melted butter into the cookie crumbs. Stir until all the crumbs are coated. Press the crumb mixture into the bottom and up the sides of a 9-inch pie dish. Chill in the fridge.
Step 2: Melt the Chocolate - In a heatproof bowl, add the semi-sweet chocolate chips and the tablespoon of butter. Microwave in 20-second intervals, stirring after each burst, until melted and smooth. Let cool for about 5 minutes.
Step 3: Whip the Cream - In a large mixing bowl, pour in the chilled heavy whipping cream. Add the powdered sugar and vanilla extract. Beat on medium-high speed until stiff peaks form.
Step 4: Combine the Mousse - Gently fold the melted chocolate into the whipped cream until no streaks remain.
Step 5: Fill the Pie Crust - Spoon the chocolate mousse filling into the crust, spreading it out evenly.
Step 6: Chill the Pie - Cover the pie loosely with plastic wrap and refrigerate for at least 4 hours, or overnight.
Step 7: Add the Whipped Cream and Garnish - Top the pie with whipped cream and sprinkle with extra chocolate chips or shavings before serving.
