In a medium bowl, combine the crushed chocolate sandwich cookies and melted butter until the mixture resembles wet sand.
Press the cookie mixture evenly into the bottom of a 9-inch (23 cm) springform pan to form the crust. Place in the refrigerator to chill while preparing the filling.
In a heatproof bowl, melt the chopped dark chocolate over a double boiler or in short bursts in the microwave, stirring until smooth. Set aside to cool slightly.
In a large mixing bowl, beat the softened cream cheese until smooth and creamy using an electric mixer on medium speed.
Add the powdered sugar, cocoa powder, and vanilla extract to the cream cheese and continue to beat until fully combined.
Slowly pour the melted chocolate into the cream cheese mixture, mixing on low speed until well incorporated.
In a separate bowl, whip the heavy cream until stiff peaks form.
Gently fold the whipped cream into the chocolate cream mixture until fully combined and smooth.
Pour the chocolate cream filling over the chilled cookie crust in the springform pan and spread evenly.
Cover the cake with plastic wrap and refrigerate for at least 4 hours, or until firm.
Once set, remove the cake from the springform pan, slice, and serve chilled.