Wickedly rich chocolate butterscotch cheesecake

To make butterscotch, chop butter into medium saucepan, add sugar and cream; stir over low heat until smooth. Simmer, uncovered, 3 minutes. Reserve 1½ cups for serving; transfer the remaining mixture into a small heatproof bowl. Refrigerate both mixtures.

Preheat oven to 150°C/300°F. Grease 22cm (9-inch) closed springform pan; line base with baking paper. Place pan on an oven tray.

Drag a large sharp knife across the back of chocolate block to make large curls. (If chocolate is hard and the curls are brittle, microwave the whole block on MEDIUM (75% power) for about 20 seconds.) You will need 100g chocolate curls. Place curls in an airtight container; keep at room temperature for serving. Chop 100g of remaining chocolate coarsely.

Combine remaining 150g chocolate and chopped butter in small saucepan; stir over low heat until smooth. Transfer to medium bowl; cool 5 minutes. Stir brown sugar and one of the eggs into chocolate mixture, then stir in sifted flour.

Spread mixture into pan. Bake brownie base for 15 minutes.

Meanwhile, beat cream cheese, extract and caster sugar in small bowl with electric mixer until combined. Beat in remaining eggs, one at a time, then beat in sour cream until combined. Pour half the cream cheese mixture over brownie base, drizzle with half the butterscotch; gently swirl through the mixture. Sprinkle with half the chopped chocolate. Repeat layers.

Bake cheesecake about 45 minutes or until barely set in the centre. Cool cheesecake in oven with door ajar. Cover cheesecake; refrigerate about 4 hours or until firm.

Stand cheesecake at room temperature for 1 hour before serving. Top cheesecake with chocolate curls. Heat reserved butterscotch in small saucepan; serve with cheesecake.