Line a 20 cm (8 inch) springform pan with parchment paper.
In a medium bowl, combine the crushed chocolate sandwich cookies and melted butter until the mixture resembles wet sand.
Press the cookie mixture firmly into the bottom of the prepared pan to form the crust. Place the pan 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 with an electric mixer until smooth and creamy.
Add the powdered sugar, vanilla extract, and sifted cocoa powder to the cream cheese and mix until fully incorporated.
Pour the melted dark chocolate into the cream cheese mixture and blend until smooth.
In a separate bowl, whip the heavy cream until stiff peaks form.
Gently fold the whipped cream into the chocolate cream cheese mixture until fully combined and smooth.
Pour the chocolate filling over the chilled cookie crust in the springform pan and smooth the top with a spatula.
Cover the cake with plastic wrap and refrigerate for at least 4 hours or until firm.
Before serving, garnish the top of the cake with grated semi-sweet chocolate.
Carefully remove the cake from the springform pan, slice, and serve chilled.