1 Heat the oven to 180°C (160° fan) 350°F gas 4. Butter a 23 cm| 9"square tin.
2 Put the plain and white chocolates into two separate heatproof bowls and add half of the butter to each. Melt over a pan of simmering (not boiling) water. Stir until the chocolate and butter are blended.
3 Add 150 g sugar and 2 beaten eggs to each bowl, then beat until smooth.
4 Stir 50 g of the flour into the plain chocolate mixture and the remaining 90 g into the white mixture.
5 Spoon tablespoons of the mixture into the tin, alternating plain and white chocolate to make a patchwork pattern. When the base of the tin is covered, spoon more mixture over the first layer, spooning white on top of the dark mixture. Drag a skewer through the tin several times to create a marbled effect.
6 Bake for 35 minutes, until the middle is just set and the white chocolate patches are pale golden on top. Leave to cool completely in the tin before cutting into squares.