1 - Preheat the oven to 175∘C. Grease a baking pan measuring roughly 12 x 8 inches.
2 - Combine the flour, cocoa, walnuts, baking powder and salt. Cream together the margarine and the brown and white sugars. Add the egg together with a little bit of vanilla essence and stir. Add the flour mixture and stir until all of the ingredients are moistened.
3 - Pour the batter into the baking tray, spread smoothly and bake for about 20 to 25 minutes in the centre of the oven. Melt the 2 chocolates in separate pans/or in the microwave.
4 - Remove the chocolate bars from the oven, let them cool and then carefully cut in half to (through the centre) to make 2 layers. Spread the white chocolate over the bottom layer and sprinkle with the chopped nuts. Place the 2nd layer on top and decorate by dribbling the dark chocolate and remaining white chocolate over the top.