Preheat oven to 180 degrees C, grease a 9 inch (23 cm) springform pan and cover the exterior with 2-3 layers of aluminium foil to ensure no water gets in.
Melt the butter, mix thoroughly with crushed biscuits and press evenly to cover the bottom of the pan. Chill while preparing the filling.
Ensure that all the filling ingredients are at room temperature before you begin. Using an electric beater or food processor, beat the cream cheese till smooth and free of lumps (3-4 minutes).
Then beat in the ricotta and sugar till well blended and smooth, followed by the cornflour.
Now beat in the eggs one at a time, and finally the vanilla extract.
Melt the chocolate gently in a double boiler adding cream as required to loosen the melted chocolate to a thick pouring consistency. Cool chocolate to room temperature.
Pour the filling into the prepared springform pan and using a tablespoon, gently pour the chocolate in concentric circles starting from the outer side. Then use a this knife to draw straight lines from the centre of the innermost circle to the outermost to create a marbling pattern or just makes waves resembling swirls of chocolate and white filling.
Place the pan in the centre of a roasting tin with water reaching the halfway mark of the pan and bake for 50 minutes to an hour or maybe a little more till the top is lightly browned and the centre is slightly wobbly.
Remove the pan from the roasting tin and cool the cake in it before chilling overnight in the fridge. Un-mould the chilled cake and serve with additional chocolate sauce if desired.