Prepare the pan: Lightly grease a 9-inch springform pan and line the bottom with parchment paper if desired.
Make the crust: Crush 24 Oreo cookies into fine crumbs. Mix with the melted butter until evenly combined. Press the mixture firmly into the bottom of the pan. Refrigerate while preparing the mousse
Mix the cream cheese: Beat the softened cream cheese and sugar until smooth and creamy. Add the vanilla extract and mix until combined.
Add the Oreos: Stir the finely crushed Oreo cookies into the cream cheese mixture until evenly distributed.
Whip the cream: Beat the cold heavy whipping cream in a separate bowl until medium to firm peaks form. Be careful not to overwhip.
Make the mousse: Fold about one-third of the whipped cream into the cream cheese mixture. Gently fold in the remaining whipped cream until smooth and fluffy
Fill the pan: Spread the Oreo mousse evenly over the chilled crust. Smooth the top with a spatula
Prepare the chocolate topping: Place the chocolate chips in a heatproof bowl. Warm the heavy cream until steaming but not boiling. Pour it over the chocolate and let it sit for 1 to 2 minutes. Stir until smooth and glossy.
Top the cake: Allow the chocolate mixture to cool slightly, then spread it gently over the mousse. Add chopped Oreos or Oreo crumbs while the topping is still soft.
Chill: Cover and refrigerate for at least 6 hours, preferably overnight.
Serve: Carefully remove the springform pan. Slice with a sharp, warm knife and garnish with whipped cream or additional Oreo pieces if desired.
