01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, allowing an overhang for easy removal.
02 - In a heatproof bowl over simmering water, melt the chocolate and butter together, stirring until smooth. Remove from heat and whisk in cocoa powder until fully incorporated.
03 - In a large mixing bowl, whisk together granulated sugar, light brown sugar, eggs, and vanilla extract until thick and pale, approximately 2 minutes.
04 - Gradually add the melted chocolate mixture into the egg mixture, whisking continuously until fully combined.
05 - Sift in flour and salt, folding gently with a spatula until just combined to avoid overmixing.
06 - Gently fold in three-quarters of the chopped walnuts, reserving the remainder for topping.
07 - Pour batter into the prepared pan and smooth 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 should have moist crumbs attached.
09 - Allow brownies to cool completely in the pan on a wire rack. Lift out using the parchment overhang, slice into squares, and serve.