In the bowl of an electric mixer fitted with the whisk attachment, combine the heavy cream, mascarpone, sugar, cocoa powder and vanilla. Mix on low speed to combine and then slowly raise the speed, until it forms firm peaks.
To assemble the cake, arrange chocolate chip cookies flat in an 9-inch springform pan, covering the bottom and breaking the cookies if needed to fill the space.
Spread 1/3 of the whipped cream evenly over the cookies. Place another layer of cookies on top, lying flat and touching, followed by another 1/3 of the cream.
Layer the cookies and cream until there are 3 layers of each, ending with a layer of cream. Smooth the top, cover with plastic wrap, and refrigerate overnight. *Push the plastic down so it is laying flat on the top layer of cream.
Run a small sharp knife around the outside of the cake and remove the sides of the springform pan. Leave the cake on the pan bottom.
Sprinkle the top with the chocolate, cut in wedges, and serve cold.