1.Grease a deep 22cm (9in) round cake pan; line base and side with baking paper, extending the paper 5cm (2in) over side.
2.Process biscuits until fine. Add butter; process until combined. Press mixture evenly over base of pan. Refrigerate for 30 minutes.
3.Meanwhile, heat cream in a small saucepan over low heat for 1 minute or until hot but not boiling. Remove from heat; stir in chocolate until melted. Cool for 15 minutes.
4.Preheat oven to 160°C/325°F. Beat cream cheese, sugar and vanilla in a medium bowl with an electric mixer until smooth (do not overbeat). Beat in eggs, one at a time, until combined; beat in sour cream.
5.Spread half the caramel over biscuit base. Pour cream cheese mixture over caramel. Drop spoonfuls of chocolate mixture and remaining caramel onto cream cheese mixture. Use a bamboo skewer to swirl mixtures together.
6.Bake for 50 minutes or until just set around the edge, but still slightly wobbly in the centre. Cool cheesecake in oven with door ajar. Refrigerate for 4 hours or overnight. Stand at room temperature for 30 minutes before serving.
Whisk caramel before using.
Swap butternut snap biscuits with plain or chocolate biscuits if you prefer. If you use another type of biscuit, add enough extra melted butter until the crumbs come together; you may need 125g (4oz).
Use a hot dry knife to cut the cheesecake, wiping the blade clean between cuts.
The cheesecake can be made a day ahead; keep covered, in the fridge.
Serve topped with whipped cream or ice-cream and grated chocolate.