Whizz the biscuits in a food processor until they are a fine crumb, then combine with the butter. Press into a lined springform cake tin and put in the fridge to chill for 1 hour.
Put your marshmallows on a greased tray and place under a hot grill for 2 mins, until toasted and bubbly. Leave to cool.
Put your cream cheese and icing sugar in a bowl and whisk with electric beaters until smooth. Whip your cream separately then fold through your mix. Add your marshmallows and whisk again until just combined.
Put your cream in a jug and put in the microwave for 1 minute, or until hot. Leave to stand for 1 min, then pour over your chopped chocolate and stir until smooth. Pour half of the mixture over your biscuit base and leave to set in the fridge for 1 hour.
Pour your marshmallow cream mixture over the top and smooth the surface. Chill in the fridge ideally overnight, or for at least 4 hours.
Gently warm your remaining ganache and pour over the cheesecake, then your remaining biscuits. Put in the fridge to chill for 1 hour.