01 - Preheat the oven to 350°F. Line an 8x8-inch baking pan with parchment paper, allowing an overhang for easy removal.
02 - Place chopped chocolate and cubed butter in a heatproof bowl over simmering water. Stir until smooth, remove from heat, and whisk in cocoa powder until fully incorporated.
03 - Whisk granulated sugar, brown sugar, eggs, and vanilla extract in a large bowl until thick and pale, approximately 2 minutes.
04 - Gradually whisk the melted chocolate mixture into the egg mixture until thoroughly combined.
05 - Sift in flour and salt, then gently fold with a spatula until just combined to avoid overmixing.
06 - Fold three-quarters of the toasted walnuts into the batter, reserving the remainder for the topping.
07 - Pour the batter into the prepared pan, smoothing the surface, then evenly sprinkle with the remaining walnuts.
08 - Bake for 22 to 25 minutes until edges are set and the center is slightly soft. A toothpick inserted should have moist crumbs.
09 - Allow the baked item to cool completely in the pan on a wire rack. Use the parchment overhang to lift it out, slice into 16 squares, and serve.