01 - Preheat the oven to 350°F. Line a 9 x 9 inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a heatproof bowl set over a saucepan of simmering water, melt the chocolate and butter together, stirring until smooth. Remove from heat and let cool slightly.
03 - Whisk in both granulated and brown sugars until glossy and thoroughly combined.
04 - Add eggs one at a time, whisking well after each addition. Stir in the vanilla extract and salt.
05 - Sift in the flour and cocoa powder. Fold gently with a spatula until just combined; do not overmix.
06 - Fold in the chopped walnuts, reserving a handful for sprinkling on top.
07 - Pour the batter into the prepared pan and smooth the top. Sprinkle with reserved walnuts.
08 - Bake for 28–32 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs (do not overbake for fudgy texture).
09 - Cool completely in the pan before lifting out and cutting into squares.