Crush the chocolate sandwich cookies into fine crumbs. Combine them with the melted butter until evenly moistened.
Press the cookie mixture firmly into the bottom of a 9-inch springform pan. Refrigerate while preparing the filling.
Melt the semi-sweet chocolate gently in the microwave or over a double boiler. Stir until smooth and let it cool slightly.
Beat the softened cream cheese until smooth. Add the powdered sugar, vanilla, and cocoa powder, then mix until creamy and well combined.
Pour in the slightly cooled melted chocolate and mix on low speed just until incorporated.
In a separate bowl, whip the cold heavy cream until medium to stiff peaks form.
Gently fold the whipped cream into the chocolate cream cheese mixture in two or three additions. Avoid overmixing.
Spoon the filling over the chilled cookie crust and smooth the surface with a spatula.
Cover and refrigerate for at least 6 hours, preferably overnight, until firm.
For the topping, whip the heavy cream, powdered sugar, and vanilla until firm peaks form. Spoon or pipe it over the chilled cheesecake.
Garnish with chocolate shavings or curls.
Carefully release the cheesecake from the springform pan. Slice with a sharp, warm knife for clean cuts and serve chilled.
