Preheat the oven to 180°C.
Mix crushed bourbon biscuits with melted butter and press into a 20cm spring-form tin.
Melt 100g of dark chocolate in a bowl over simmering water.
Blend cream cheese, ricotta, and sugar in a food processor until smooth.
Add eggs, cocoa powder, melted chocolate, and crumbled Christmas pudding to the mixture.
Spoon the mixture onto the base and bake for 45 minutes until springy.
Cool the cake in the tin, then remove and top with grated chocolate and cocoa powder.
Melt remaining chocolate and cream together, then whisk in butter to make the sauce.
Serve slices with the sauce and a dollop of clotted cream.