Preheat the oven to 180°C, (160°C for fan ovens), Gas Mark 4. Double wrap the outside of the cake tin in clingfilm.
Mix the crushed biscuits in a bowl with the cocoa and buttery spread thoroughly then lightly press into the base of the tin. Chill for 10 minutes.
Whisk the cream cheese, crème fraîche, sugar, cornflour, eggs and vanilla (use an electric hand whisk or blender if you have one). 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 halfway 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. Cool and then chill for 4 hours or overnight. Remove from the tin and serve.
The secret to the sticky texture of this cheesecake is in the gentle cooking in a water bath (roasting tin filled with hot water). Cooling the cheesecake slowly in the oven helps to prevent the top cracking. Try serving this with blueberry compote and more half fat crème fraîche.