1. Butter a baking pan 33 x 23 cm and place a greased parchment paper on it. Preheat oven to 180˚C. In a bain-marie, over slowly simmering water, melt the butter together with the chocolate. Let the mixture slightly cool. Whisk the eggs in a large bowl or mixer. Whisk in the salt, sugar, brown sugar, and vanilla.

2. Whisk in the chocolate mixture and add flour. Stir gently until all ingredients are very well combined. If using chopped walnuts, add them to the mixture. Pour the mixture into the prepared baking pan. If using whole walnuts, place them on top of the mixture. Bake the brownie for about 35-40 minutes or until it begins to shine and its crust on the surface cracks. Let it cool inside the pan, placed on a rack.