01 - Preheat oven to 350°F. Line an 8x8 inch baking pan with parchment paper, allowing overhang on sides for easy removal.
02 - Set heatproof bowl over simmering water. Add chopped chocolate and butter, stir until completely smooth and combined. Remove from heat, cool briefly.
03 - Whisk granulated sugar and brown sugar into melted chocolate mixture until fully dissolved and uniform.
04 - Beat in eggs one at a time, ensuring each is fully incorporated before adding the next. Stir in vanilla extract.
05 - In separate bowl, sift together flour, cocoa powder, and salt until well blended.
06 - Gently fold dry mixture into wet ingredients until just combined. Do not overmix—some flour streaks may remain.
07 - Carefully fold chopped walnuts into batter, distributing evenly throughout.
08 - Pour batter into prepared pan. Spread evenly with spatula, smoothing top surface.
09 - Bake 30-33 minutes until toothpick inserted in center emerges with moist crumbs, not wet batter.
10 - Cool completely in pan on wire rack. Lift out using parchment overhang, cut into 16 squares.