Preheat oven to 180 degrees C. In a large bowl, mix together the biscuit crumbs, icing sugar and 4 tablespoons cocoa. Add melted butter and stir until well mixed.
Pat into the bottom of a 23cm cake pan with removable base so you can remove it easily.
Bake in preheated oven for 10 minutes; set aside. Increase oven temperature to 230 degrees C.
In a large bowl, combine cream cheese, caster sugar, 3 tablespoons cocoa and flour. Beat at medium speed until well blended and smooth. Add eggs one at a time, mixing well after each addition.
Blend in the mascarpone or sour cream and Baileys liqueur mixing on low speed. Pour filling over biscuit base.
Bake in preheated oven for 10 minutes. Reduce oven temperature to 120 C and continue baking for 60 minutes.
With a knife, loosen cake from sides of tin. Allow to cool slightly, then push up base of the pan to remove cheesecake. Chill before serving. If your cake cracks, a helpful tip is to dampen a palette knife and smooth the top, then sprinkle with some biscuit crumbs.