For the base: Whiz all the ingredients in a food processor until you have moist crumbs. Tip into a greased 23cm springform cake pan and press firmly over the base, and 5cm up the sides. Refrigerate while making the filling.

For the filling: Heat the oven to 180C. Heat the chocolate and half the sour cream in a bowl set over barely simmering water, stirring until smooth.

Using an electric mixer, beat in the cream cheese, vanilla extract, sugar and remaining sour cream until smooth. Beat in the flour and zest, then the eggs one at a time.

Place the cake tin on a baking tray, and pour two-thirds of the batter into the crust. Whisk the melted chocolate cream into the remaining batter until smooth. Using a soup ladle, pour the chocolate filling over the vanilla filling until you reach the top of the crust (you may not use it all).

Bake for 50 minutes or until the centre is just firm and the top is lightly golden - it will split slightly. Cool, then refrigerate overnight before serving.

To serve: Dust with cocoa powder, use a hot knife to slice the cheesecake and serve.