Heat the oven to 140ºc / Gas 1. Grease a 20cm springform cake tin.
Melt the butter in a saucepan, then stir together the biscuit crumble until all the biscuit is covered in butter. Press into the base of the tin so that it's all level and bake in the oven for 10mins. When baked, remove from the oven and set aside. Do not turn the oven off.
Break the chocolate into pieces and melt in a heatproof bowl over a pan of simmering water. Once melted remove from the heat.
In a separate bowl, beat together the cream cheese, creme fraiche, sugar, eggs and egg yolk until well blended. Put half the mixture into a separate bowl and stir in the melted chocolate.
Fill the tin with alternate spoonfuls of the plain and chocolate mixtures. Drag a skewer or cocktail stick through the mixture to create a marbled pattern.
Bake for about 50mins, until the filling has set but still has a slight wobble. Switch off the oven and leave the cheesecake inside, with the door slightly open to cool completely.