Preheat the oven to 160°C/fan 140°C/gas mark 3. Grease a 23cm round springform tin with a little melted butter and line with baking parchment.
Crush the biscuits and mix with the melted butter. Press into the base of the tin and leave to set in the fridge.
Meanwhile, gently melt the chocolate and chocolate thins together. Put the sugar, soft cheese, mascarpone and eggs in a mixing bowl and pour in the melted chocolate. Beat well with a spoon. Gently tap the bowl on the work surface to remove air bubbles.
Pour on to the biscuit base and decorate with the clementine slices. Place the tin on a baking sheet and bake for 1 hour until set. Turn off the oven and leave to cool with the door ajar. Chill until ready to serve.