01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving enough overhang on two sides to lift the brownies out later.
02 - Combine the butter and chopped dark chocolate in a heatproof bowl set over a saucepan of simmering water. Stir gently until completely melted and smooth, then remove from heat and let cool for about 5 minutes.
03 - Add the granulated sugar and light brown sugar to the melted chocolate mixture. Whisk until fully combined and the mixture appears glossy.
04 - Crack in the eggs one at a time, whisking thoroughly after each addition until fully incorporated. Stir in the vanilla extract.
05 - Sift the all-purpose flour, unsweetened cocoa powder, and fine salt together directly into the wet mixture. Use a spatula to fold gently until just combined — avoid overmixing to keep the texture tender.
06 - If desired, fold in the chopped walnuts or pecans and dark chocolate chips until evenly distributed throughout the batter.
07 - Pour the batter into the prepared baking pan and use a spatula to spread it evenly, smoothing the surface.
08 - Bake on the center rack for 22–25 minutes, or until the top is set and slightly crackled while the center still has a gentle jiggle. Do not overbake.
09 - Allow the brownies to cool completely in the pan on a wire rack. Once fully set, use the parchment overhang to lift them out and cut into 12 even squares.