Prepare the Chocolate Base: Crush chocolate cookies into fine crumbs, mix with melted butter, press into a springform pan, and chill.
Make the Dark Chocolate Mousse: Dissolve gelatin in warm water, melt dark chocolate, whip cream and fold into chocolate, pour over base, and refrigerate for 30 minutes.
Prepare the Milk Chocolate Mousse: Repeat steps using milk chocolate, spread over dark mousse, and refrigerate for 30 minutes.
Make the White Chocolate Mousse: Repeat steps using white chocolate, pour over milk mousse, and chill for at least 6 hours or overnight.
Garnish and Serve: Remove from springform pan, dust with cocoa powder or decorate with berries, slice, and enjoy.
Notes
For a vegetarian version, substitute gelatin with agar-agar. Chill each layer for a distinct separation.