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