Amaretto Cheesecake

Ingredients
2 cups crushed digestive biscuits
1 tbs good-quality cocoa powder, sifted
3 oz unsalted butter, melted
4 egg yolks
1 tsp vanilla extract
1 cup caster sugar
1.5 pounds ricotta cheese
1/2 cup sour cream
1/4 cup amaretto*
1/4 cup plain flour
1/2 cup thickened cream, whipped
3 eggwhites
7 oz good-quality dark chocolate, chopped

Directions:
Preheat the oven to 325 F. Lightly grease a 9 inch springform pan. Cover the outer base and sides of the pan with a double layer of foil. Combine the biscuit crumbs, cocoa and butter, then spread the mixture over the base and 3/4 of an inch up the sides of the prepared pan.
Process the egg yolks, vanilla and sugar in a food processor. Add the ricotta, sour cream, amaretto and flour, and process until smooth. Place the mixture in a mixing bowl and fold through the whipped cream. Beat the eggwhites until soft peaks form, then fold into mixture, making sure it's well combined. Finally, fold in half the chocolate. Place in prepared pan, then sit in a deep baking dish. Pour boiling water into baking dish so it comes halfway up sides of pan. Bake for 1 hour, turn the oven off and leave to cool completely, then refrigerate for a minimum of 1 hour.
To serve, place the remaining chocolate in a bowl over a pan of simmering water (making sure the bowl doesn't touch water) and stir until chocolate melts. Cool slightly, then serve slices of cheesecake drizzled with the melted chocolate.