Preheat the oven to 180 C / Gas 4. Lightly butter a 25cm springform cake tin.
Combine the digestive biscuit crumbs with the cocoa powder and 200g sugar. Mix with melted butter. Press into the bottom and 3 to 5cm up the sides of the prepared springform tin.
Bring 400g sugar and the water to a boil in a large, heavy-bottomed saucepan. 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 double cream (the mixture will bubble vigorously). Return the toffee mixture to the hob over low heat, and stir until completely liquefied. Stir in the chocolate until melted, then stir in the soured cream. Remove from the heat and set aside.
Beat the cream cheese until fluffy with an electric mixer. Add toffee 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 tin.
Place tin into a hot water bath with the water coming halfway up the sides of the springform tin. Bake in the centre of the preheated oven until the centre has set, about 1 hour 15 minutes. The cheesecake is done when the middle is still a little wobbly. Chill completely before serving.