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