Preheat oven to 180 degrees C. Grease or coat a 25cm springform pan with nonstick cooking spray.
Combine the biscuit crumbs with the cocoa powder and the sugar. Toss with melted butter. Press into the bottom and up the sides of the prepared springform pan.
Bring 2 cups sugar and the water to a boil in a large, heavy-bottomed pot. Reduce heat to medium-low, and continue to cook without stirring until the mixture has turned a deep, golden colour, 30 to 45 minutes. Remove from the heat, and carefully pour in the heavy cream (the mixture will bubble vigorously). Return the caramel to the stove over low heat, and stir until completely liquefied. Stir in the chocolate until melted, then stir in the sour cream. Remove from heat and set aside.
Beat the cream cheese until fluffy in an electric mixer. Add caramel mixture, and blend well, scraping the sides of the bowl occasionally. Add eggs, one at a time, scraping the bowl after each addition. Add the vanilla with the last egg. Mix until well incorporated, then pour mixture into prepared pan.
Place pan into a baking tray with the water coming half way up the sides of the springform pan. Bake in the centre of the preheated oven until the centre of the cheesecake has set, about 1 hour 15 minutes. The cheesecake is done when the middle is still a little wobbly. Let it sit in the oven till cool. Chill completely before serving.