Preheat the oven to 180 C / Gas 4. Grease a 23cm springform cake tin and line the sides with baking parchment.
Combine the biscuits, chocolate, sugar and butter and mix well. Press into the base of the prepared cake tin.
Bake in the oven at 180 C / Gas 4 for 10 minutes. Remove from the oven and allow to cool. Increase the oven temperature to 230 C / Gas 7.
For the filling:
Meanwhile, combine the cream cheese, caster sugar, orange zest, orange juice and flour in a large bowl. Beat at a medium speed until well blended and smooth. Add in the eggs, one at a time, mixing well after each addition. Remove the base from the fridge then spoon the filling over, smoothing the top.
Bake in the oven at 230 C / Gas 7 for 10 minutes then reduce the temperature to 120 C / Gas 1/2 and continue baking for 50 minutes.
For the topping:
While the cheesecake is baking, make the topping. Combine soured cream, grated chocolate and caster sugar together. Pour on top of the cake and continue baking for a further 20 minutes.
Remove from the oven and loosen the cake from the sides of tin. Allow to cool slightly, then push up the base of the tin to remove cheesecake and discard the baking parchment. Cool to room temperature for 1 hour and then chill in the fridge for 2 hours. Before serving, transfer to a serving plate and place an orange twirl on top.