1-Preheat your oven to 350°F (175°C): Preheat your oven to 350°F (175°C). Crush the chocolate sandwich cookies in a food processor until fine crumbs form. Add the melted butter and pulse until the mixture resembles wet sand. Press the mixture firmly into the bottom and up the sides of a 9-inch pie dish, using the bottom of a glass or measuring cup to compact it tightly. Bake for 8-10 minutes, then let it cool completely.
2-In a heatproof bowl, melt the chocolate: In a heatproof bowl, melt the chocolate chips and heavy cream together over a double boiler or in the microwave in 20-second increments, stirring until smooth. In a separate bowl, whisk together the sugar and eggs until well combined. Slowly pour the melted chocolate mixture into the egg mixture, whisking constantly. Stir in the vanilla extract. Pour the fudge mixture over the cooled crust and bake at 325°F (163°C) for 15-18 minutes, until the edges are set but the center is slightly wobbly. Cool completely.
3-Melt the chocolate chips with ½ cup: Melt the chocolate chips with ½ cup of heavy cream in the microwave or over a double boiler, stirring until smooth. Let it cool slightly. Whip the remaining 1 cup of heavy cream with the powdered sugar until stiff peaks form. Gently fold the whipped cream into the cooled chocolate mixture. Spread the mousse evenly over the cooled fudge layer. Chill in the refrigerator for at least 3 hours, or until set.
4-Whip the heavy cream, powdered sugar, and: Whip the heavy cream, powdered sugar, and vanilla extract together until stiff peaks form. Spoon or pipe the whipped cream over the chocolate mousse layer. For a decorative finish, use a piping bag fitted with a star tip.
5-Decorate the top with chocolate shavings, curls,: Decorate the top with chocolate shavings, curls, or pieces. Lightly dust the top with cocoa powder, if desired, for a polished look.