1/2 lb unsalted butter
1/2 lb semi-sweet chocolate, cut into 1/4 inch pieces
4 eggs
1/4 tsp salt
1/2 lb superfine sugar
1/2 lb dark brown sugar
1/2 tbsp vanilla extract
1/4 pound all purpose flour
1 cup walnuts

Method:

• Take a medium-sized baking tray and line it with butter paper.
• Preheat the oven at 180 degree centigrade.
• In a saucepan bring water to a boil and turn off the heat.
• In a glass bowl, place the chocolate and set it over or partially submerge it in the pan of hot water.
• Stir occasionally and add butter and mix together until melted completely.
• Whisk the eggs in a large bowl and add salt, sugar, brown sugar and vanilla, whisking them well together.
• Stir in the chocolate and butter mixture, then fold flour using a spoon.
• Stir in 3/4 cup of walnut pieces.
• Pour batter into the prepared pan and spread evenly.
• Sprinkle with the remaining walnut pieces and bake for about one hour, until the top has formed a shiny crust and the batter is moderately firm.
• Cool and then remove the paper and cut into 2-inch squares.
• Dust lightly with coco powder and serve.