Baked toffee cheesecake

Preheat the oven to 180°C, 350°F, Gas Mark 4. Double wrap the cheesecake tin in clingfilm. To make the base; place the crushed biscuits into a bowl and add the cocoa and melted margarine or butter, mix thoroughly then lightly press into the base of the tin. Chill. Place the cream cheese, crème fraîche, sugar, cornflour, eggs and vanilla into a food processor and blend for 2-3 minutes, stopping to scrape the sides of the processor occasionally. Pour the filling over the biscuit base. Sit the cheesecake in a roasting tin. Beat the caramel in a small bowl and then gently swirl into the creamy filling. Pour very hot water into the roasting tin to come 2.5cm (1in) up the sides of the cake tin. Bake in the centre of the oven for 45 minutes. Turn off the oven, without opening the door, and leave for 1 hour to set. Remove to a rack to cool then chill for 4 hours or overnight. Remove from the tin and serve.