Prepare the tin and oven

• Grease and line the base and sides of a springform pan with baking paper.
• Preheat the oven to 170 °C.

Make the brownie batter

• Melt the dark chocolate with the butter in a saucepan over low heat, then let it cool slightly.
• Whisk the sugar and eggs until pale and thick, then stir in the melted chocolate mixture.
• Fold in the sifted flour, cocoa powder and salt until just combined.

Bake the brownie base

• Pour the brownie batter into the prepared pan and spread it evenly.
• Bake until the surface is just set, then leave to cool completely.

Make the Oreo cheesecake filling

• Beat the cream cheese with the sugar until smooth.
• Add the double cream, vanilla extract and cornflour and beat until thick and creamy.
• Fold in the crushed Oreo biscuits.

Bake the cheesecake

• Spread the cheesecake filling over the cooled brownie base and level the top.
• Bake until the edges are set and the centre still has a slight wobble.
• Turn off the oven and leave the cheesecake inside with the door ajar to cool slowly.

Chill and serve

• Chill the cheesecake in the fridge until fully set.
• Release it from the pan, slice and serve.