Grease and line an 18cm (base measurement) springform pan. To make the base, place the cookies in a food processor or blender and process until it resembles coarse breadcrumbs. Add butter and continue to process until well combined.

Remove the crumb mixture and, using your hands, press into the base of the prepared pan, until the surface is smooth and even. Cover and chill for 20 minutes.

Meanwhile, place the chocolate in a small heatproof bowl over a saucepan half-filled with simmering water (make sure the bowl doesn't touch the water). Use a metal spoon to stir occasionally until the chocolate melts and is smooth. Remove from the heat and cool to room temperature.

Place the cream cheese, ricotta, and sugar in the bowl of an electric mixer and beat for a couple of minutes or until well combined. Add the thickened cream and continue beating for 5 minutes or until mixture is smooth and thick.

Pour the mixture into the pan over the biscuit base and smooth the surface. Drizzle with the melted chocolate and swirl lightly with a knife to create ripple effect. Freeze for 2 hours or until solid.

To serve, carefully remove the cheesecake from the pan. Serve immediately with cream or berries.