Line an 18cm loose-bottomed cake tin with baking parchment. Whizz the biscuits to crumbs in a food processor. In a bowl, mix together the biscuit crumbs, the melted butter and cocoa powder then press into the base of the tin.
Preheat the oven to 180°C, fan 160°C, gas mark 4. Place the dark and white chocolate in separate heatproof bowls and melt over 2 pans of gently simmering water.
Meanwhile, beat together the cheese, soured cream, sugar, eggs and vanilla with a wooden spoon, until combined.
Divide the mixture bewteen 2 bowls, then stir the melted dark chocolate into one and the melted white chocolate into the other.
Pour the dark chocolate mixture into the cake tin, then pour the white chocolate mixture over the top. Lightly drag a knife or cocktail stick through both mixtures to create a swirly pattern.
Bake for 30 minutes, until just set but still slightly wobbly in the centre. Cool in the tin then serve with the raspberries.