Heat oven to 150C, 130C fan, 300F, gas 2. For the base place the biscuits, butter and mixed spice in a mixing bowl, mix well then press into the base of the tin. Bake in the oven for 10 minutes, then remove and cool whilst you make the filling.
Melt the chocolate in a bowl over a pan of gentle simmering water, do not allow the base of the bowl to touch the water, stir occasionally then remove from the heat.
Place the cream cheese, sugar, eggs, cornflour, coffee and sour cream in a large mixing bowl and beat until smooth. Fold in the warm melted chocolate, stir well then finally add the rum, make sure everything is thoroughly combined
Pour the cheesecake mixture over the base. Give the tin a couple of sharp taps on the worktop to remove any air bubbles then smooth the top. Place on a baking tray and bake for 45 minutes to 1 hour or until the top is set but there is still a good wobble in the centre. Remove from the oven, run a round bladed knife around the outside of the cheesecake and place the whole tin on a wire rack to cool completely. Transfer to the fridge when cool and chill for a minimum of 6 hours or overnight.
To serve, run a knife around the outside of the cheesecake and release the tin. Scatter the surface with grated chocolate, chocolate coated coffee beans and a dusting of cocoa powder. The cheesecake will keep for 3 days, covered in the fridge.