Grease and line 24cm springform cake pan.
Whizz biscuits in processor to fine crumbs. Add butter, pulse to combine, then press into the base of the cake pan. Refrigerate for 30 minutes.
Preheat oven to 170C.
Wipe out processor (no need to wash). Place cream cheese, mascarpone and caster sugar in machine, whizz to combine. Add eggs, combine, then add cocoa, chocolate and liqueur. Process until smooth. Add half the cocoa nibs, pulse to combine, then spread filling over chilled biscuit base.
Bake 45-50 minutes until cake is firm to the touch, but slightly wobbly. Turn off oven and cool cheesecake in oven with door ajar. Chill for 2-3 hours or overnight before serving.
Fold remaining cocoa nibs into whipped cream. Pile on top of cheesecake and serve sprinkled with extra nibs.