Step 1

Grease a 23cm springform cake pan. For the base, process biscuit, butter, cocoa powder and a pinch of salt in a food processor until you have moist crumbs. Tip into the cake pan and press firmly over the base and 5cm up the sides. Refrigerate while making the filling.

Step 2

Preheat the oven to 180C.

Step 3

Heat chocolate and half the sour cream in a bowl set over a saucepan of barely simmering water, stirring until smooth.

Step 4

Using an electric mixer, beat the softened cream cheese, vanilla extract, caster sugar and remaining sour cream until smooth. Beat in the plain flour and orange or lemon zest, then beat in the eggs one at a time. Pour two-thirds of the cream cheese mixture into the crust.

Step 5

Whisk the melted chocolate cream into the remaining cream cheese mixture until smooth. Using a soup ladle, pour the chocolate filling over the vanilla filling until you reach the top of the crust.

Step 6

Place in the oven and bake for 40 minutes, until the centre is just firm and the top is lightly golden – it will split slightly. Cool, then refrigerate overnight before serving. Dust with cocoa powder, use a hot knife to slice the cheesecake and serve.