Grease and line the base and sides of a 20cm / 8 inch loose-bottomed cake tin with non-stick baking paper. Set aside.
Tip the gingernut biscuits into a food processor and blitz into fine crumbs. Add the butter and whizz again until you have the desired crumbly consistency.
Tip the biscuit mixture into the prepared tin and pat into a smooth and compact layer using the back of a spoon. Cover and chill in the fridge whilst you make the filling.
Meanwhile, melt the chocolate in a heatproof bowl set over a pan of barely simmering water (make sure the bottom of the bowl doesn’t touch the water). Stir occasionally until smooth and melted. Remove from the heat and leave to cool slightly.
Scoop the insides from the 3 passion fruits into a sieve over a bowl. Using a spoon, press down to extract as much juice as possible and catch all the seeds. Set aside.
Whisk together the cream cheese and double cream until just holding its shape. Pour in the melted chocolate and passion fruit juices, then continue to whisk until just combined.
Spoon the mixture over the biscuit base in an even layer, finishing by smoothing over the top. Cover with cling film and chill in the fridge for at least 6 hours, or until set.
To decorate: whisk the double cream until you have medium peaks. Spoon over the top of the cheesecake and decorate with the remaining passion fruit pulp and slices, if wished.