01 - Preheat oven to 350°F. Line an 8x8-inch baking pan with parchment paper, leaving overhang on two sides for easy removal.
02 - In a heatproof bowl set over a saucepan of simmering water, melt the butter and chopped dark chocolate together, stirring gently until completely smooth. 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 well blended and smooth.
04 - Add eggs one at a time, whisking thoroughly after each addition until fully incorporated. Stir in the vanilla extract until the mixture is glossy and homogeneous.
05 - Sift the all-purpose flour, unsweetened cocoa powder, and fine salt together directly into the wet mixture. Using a spatula, 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 spread evenly. Smooth the surface with a spatula for a uniform layer.
08 - Bake for 22 to 25 minutes, or until the top is set and slightly crackled while the center retains a gentle jiggle. Avoid overbaking for the best fudgy texture.
09 - Allow the 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.