- 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.