Perfect fudgy dark and white ripple brownies with raspberries

100g (3½oz) dark chocolate, broken into pieces

100g (3½oz) white chocolate, broken into pieces

150g (5oz) unsalted butter, softened

200g (7oz) caster (superfine) sugar

75g (3oz) soft light brown sugar

3 large free-range eggs

1 tsp vanilla bean paste

150g (5oz) plain (all-purpose) flour

150g (5oz) raspberries