by Diana Middleton
150g plain chocolate-covered digestive biscuits
60g butter, melted
125ml creme fraiche
3tbsp instant coffee, dissolved in 3tbsp just-boiled water
125g caster sugar, plus 1 1/2tbsp for the topping
4 eggs, beaten
240ml sour cream
cocoa powder, to dust
a 20-cm springform cake tin, greased
Put the digestive biscuits in a food processor and process until they become crumbs, then combine with the melted butter. Tip the mixture into the prepared cake tin and smooth out to make an even base. Cover and chill for 30 minutes.
Preheat the oven to 180C/350F/Gas 4. Beat together the mascarpone and creme fraiche until smooth, then stir in the coffee and sugar. Stir in the eggs until well mixed.
Wrap the base and sides of the tin in two single sheets of aluminium foil, then pour the masarcarpone mixture over the crumb base.
Put into a roasting tin and pour water around the cake tin so that it reaches half to two-thirds of the way up the sides. Bake for about 50 minutes until set but still soft.
Meanwhile, stir the remaining 1 1/2 tablespoons of sugar into the sour cream. Remove the cheesecake from the oven, gently spoon over the sour cream, spreading it out evenly, then return to the oven for 10 minutes.
Remove from the oven and leave to cool, then cover and chill for at least four hours or overnight. To serve, carefully unmould and dust with cocoa powder.