01 - Preheat oven to 350°F. Line an 8x8 inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - Place chocolate and butter in a heatproof bowl set over simmering water. Stir until completely melted and smooth. Remove from heat and let cool slightly.
03 - Whisk granulated sugar and brown sugar into the chocolate mixture until well combined.
04 - Add eggs one at a time, whisking thoroughly after each addition. Stir in vanilla extract.
05 - In a separate bowl, sift together flour, cocoa powder, and salt.
06 - Gently fold the dry ingredients into the wet mixture until just combined. Do not overmix.
07 - Fold in the chopped walnuts until evenly distributed.
08 - Pour batter into the prepared pan and smooth the top evenly.
09 - Bake for 30–33 minutes, or until a toothpick inserted in the center comes out with moist crumbs rather than wet batter.
10 - Cool completely in the pan on a wire rack. Lift out using the parchment overhang, cut into squares, and serve.