Preheat the oven to 350°F / 180°C. Crush 125g of digestive biscuits to fine crumbs and mix in 60g of softened butter and 1 tablespoon of cocoa powder.
Press into the base of a 9in / 23cm springform pan and freeze while you make the filling.
Beat 500g of softened cream cheese until smooth, then beat in 150g of superfine sugar, 1 tablespoon of cornstarch, and ½ teaspoon of vanilla extract.
Beat in 3 large eggs, 3 large egg yolks, and 150ml of sour cream. Dissolve ½ teaspoon of cocoa powder in 1 tablespoon of hot water and beat it in with 175g of melted dark chocolate.
Wrap the outside of the pan in plastic wrap, then in heavy-duty aluminum foil. Pour in the filling, set the pan in a roasting pan, and add hot water to come halfway up its side.
Bake for 45-60 minutes, until the top is set but still has a slight wobble underneath. Cool on a wire rack, then refrigerate overnight.
