Photograph: Martin Poole
Preheat the oven to 180°C, fan 160°C, gas 4. Grease an 18cm square cake tin and line it with baking paper.
Put the dark chocolate in a heatproof bowl set over a pan of barely simmering water. Melt gently, stirring now and then. Cool slightly.
Beat the butter and sugar in an electric mixer. Add the eggs, a little at a time, and continue mixing. Tip in the flour, add the melted dark chocolate and a pinch of salt; mix to combine. Fold in the chopped white chocolate and 125g of the raspberries.
In a bowl, whisk together all the ingredients for the cheesecake mixture. Tip half of the chocolate brownie mixture into the prepared tin and spread it out evenly. Add half of the cheesecake mixture to the brownie base in blobs. Gently swirl together using a skewer.
Spread over the remaining brownie mixture, then swirl in the rest of the cheesecake mixture. Bake the brownies for 50 minutes. Leave to cool in the tin before cutting into squares. Decorate with the remaining raspberries.
Get ahead: you can make these the day before; store in an airtight container.