01 - Preheat the oven to 350°F. Line an 8×8-inch baking pan with parchment paper, leaving enough overhang on the sides to lift the brownies out later.
02 - Set a heatproof bowl over a saucepan of barely simmering water. Add the butter and chopped dark chocolate, stirring gently until completely melted and smooth. Remove from heat and let cool for about 5 minutes.
03 - Pour the granulated sugar and light brown sugar into the melted chocolate mixture. Whisk until well blended and no sugar lumps remain.
04 - Add the eggs one at a time, whisking thoroughly after each addition until fully incorporated. Stir in the vanilla extract until the batter is glossy and smooth.
05 - Sift the flour, cocoa powder, and salt directly into the batter. Using a spatula, gently fold the dry ingredients into the wet mixture until just combined. Be careful not to overmix, as this will make the brownies tough.
06 - If using, fold in the chopped walnuts or pecans and dark chocolate chips with a few gentle strokes to distribute evenly throughout the batter.
07 - Pour the batter into the prepared baking pan and spread it into an even layer using a spatula, smoothing the top.
08 - Bake for 22 to 25 minutes, or until the top is set and slightly crackled while the center still has a gentle jiggle when lightly shaken. Avoid overbaking for the best fudgy texture.
09 - Remove from the oven and allow the brownies to cool completely in the pan on a wire rack. Once fully cooled, use the parchment overhang to lift the slab out and cut into 12 even squares.