1. Preheat the oven to gas mark 3. Grease a 19cm x 23cm (7 1/2in x 9 1/2in) shallow cake tin and line with baking parchment.
2. Place the plain chocolate and butter in a bowl set over a pan of simmering water and leave until melted. Remove from the heat and beat in the sugar, eggs and vanilla essence, then fold in the flour.
3. Pour the mixture into the cake tin. Spoon the melted white chocolate over the mixture. Use a skewer to swirl the white chocolate into the mixture, to create a marbled effect.
4. Bake for 30-35 mins until just firm to the touch. Leave to cool in the tin before cutting into squares.
Top tip: For a more indulgent pud, serve hot with scoops of ice-cream and warm chocolate fudge sauce. We like English Provender Company's Double Chocolate Fudge Sauce