01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving overhang on two sides for easy removal.
02 - Place butter and chopped dark chocolate in a heatproof bowl set over a saucepan of simmering water. Stir gently until completely melted and smooth. Remove from heat and let cool for 5 minutes.
03 - Add granulated sugar and light brown sugar to the melted chocolate mixture. Whisk until fully combined and glossy.
04 - Add eggs one at a time, whisking thoroughly after each addition until the batter is smooth and emulsified. Stir in vanilla extract.
05 - Sift all-purpose flour, unsweetened cocoa powder, and fine salt together into a separate bowl. Gently fold the dry mixture into the wet batter using a spatula until just combined—avoid overmixing to maintain a tender crumb.
06 - If desired, fold chopped walnuts or pecans and dark chocolate chips into the batter with a few gentle strokes.
07 - Pour the batter into the prepared baking pan and spread evenly. Smooth the surface with a spatula for a uniform finish.
08 - Bake on the center rack for 22–25 minutes, or until the top is set and glossy with a slight jiggle in the center. A toothpick inserted should come out with moist crumbs, not clean.
09 - Allow brownies to cool completely in the pan on a wire rack. Once fully set, lift out using the parchment overhang and cut into 12 even squares.