Marbled chocolate cheesecake

Method

1. For the base, whizz the biscuits, cocoa and pistachios to a sandy texture in a food processor. With the motor running, pour in the melted butter in a steady stream until well combined. Press the crumbs over the base and up the sides of the prepared cake tin to form a case. Using a knife, level off the sides of the case to give clean edges.
2. Meanwhile, for the filling, soak the gelatine in a small bowl of cold water until floppy. Put the white chocolate in a heatproof bowl with 3 tbsp of the cream and set it over, but not touching, a pan of gently steaming (not simmering) water. Leave the chocolate to melt without disturbing it, then stir until smooth. Squeeze the water from the gelatine, stir the gelatine into the chocolate, then set aside to cool a little.
3. Meanwhile, set another 3 tbsp of the cream aside and pour the rest into a large mixing bowl with the cream cheese and mascarpone. Beat together with an electric mixer until very stiff and smooth – it should lose its shine and become dull and matte. Briefly beat in the white chocolate mixture. Set aside 75g of the mixture, spoon the rest into the chilled crumb case, then smooth the top with the back of a spoon. Chill for 30 minutes.
4. Mix the reserved cream with the reserved filling mixture and set aside. Melt the reserved cream with the reserved filling mixture and set aside. Melt the dark chocolate in the same way as the white, but without cream, then remove from the heat. Spread the reserved filling mixture over the cheesecake, then drizzle over the slightly cooled dark chocolate and swirl the two together with a knife. Chill for at least 4 hours or overnight. Slice with a hot knife and serve.