Preheat oven at 350 F and line a 9 x 9 inch or 8 x 10 inch baking tray with baking paper.
In a heavy bottom pan on low heat, melt butter and chocolate.
Whisk in brown sugar and white sugar.
Transfer the mix to a mixing bowl. Add eggs, salt. Whisk lightly until combined.
Sift flour and cocoa powder in the bowl. Also add the chopped walnuts and incorporate with help of a spatula. The mixture will very thick now.
Transfer the brownie batter to the baking tray.
Bake for 25-35 minutes until the center is just set. Test by inserting a toothpick after 25 minutes. If you see liquid batter on it, cook for another 5 minutes. If you see wet crumbs on a toothpick. Immediately remove the brownie from the oven and let it cool.*30-35 minutes for a 8-inch pan. *25-30 minutes for a 9-inch pan.
Let the brownie cool completely in the pan then slice. Then, remove the brownie from the tray and slice it into 2x2 inch squares.
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Notes
Butter: Use unsalted butter for this recipe. If using salted butter, skip the salt in the recipe.
Eggs: Use room temperature eggs.
Sugar: You may use icing sugar or powdered sugar for this recipe. If you grind your own sugar, measure powdered sugar. Do Not measure granulated sugar, and then grind it afterward.
Brown sugar: If you are short on brown sugar, white sugar can do.
Cocoa Powder: Use good quality cocoa powder for the best taste like Hershey's dark cocoa.
Walnuts: Use raw walnuts. You can add extra chopped walnuts on the surface if you like.