For the cheesecake, crush the bourbon biscuits into crumbs and place into a bowl. Pour over the melted butter. Press into a 20cm/8in spring-form cake tin to form the base.
Place the chocolate in a bowl over barely simmering water and melt, making sure the water doesn't touch the bottom of the bowl.
Place the cream cheese, ricotta and sugar in a food processor and blend until smooth.
Next add the eggs, then the cocoa powder and the melted chocolate. Stir in the Christmas pudding, if using.
Spoon the mixture onto the base. Place the cake in the oven and cook for approximately 45 minutes, or until cake is springy to the touch. Allow to cool in the tin.
When cool, turn out of the tin and top with chopped or grated chocolate and a dusting of cocoa.
For the sauce, place the chocolate and cream in a clean bowl over a pan of simmering water and melt, making sure the water doesn't touch the bottom of the bowl. When melted, whisk in the butter. The sauce must be prepared just before the cake is served, because it can't be reheated.
Serve large slices with a big spoonful of sauce and a dollop of clotted cream. Dust the edge of the plate with cocoa.