01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a heatproof bowl set over simmering water, melt the chocolate and butter together, stirring until smooth. Remove from heat and whisk in cocoa powder until fully combined.
03 - In a large bowl, whisk granulated sugar, brown sugar, eggs, and vanilla extract until thick and pale, about 2 minutes.
04 - Gradually add melted chocolate mixture to the egg mixture while whisking continuously until well combined.
05 - Sift flour and salt over the mixture. Fold gently with a spatula until just combined, avoiding overmixing.
06 - Fold in three-quarters of the toasted walnuts, reserving the remainder for topping.
07 - Pour batter into prepared pan and smooth the surface. Evenly sprinkle remaining walnuts on top.
08 - Bake for 22 to 25 minutes until edges are set but center is slightly soft. A toothpick inserted should come out with moist crumbs.
09 - Allow brownies to cool completely in the pan on a wire rack. Use the parchment overhang to remove, then slice into 16 squares.