01 - Preheat oven to 350°F. Line an 8×8-inch baking pan with parchment paper, leaving an overhang on two sides for easy removal.
02 - Set a heatproof bowl over a saucepan of barely simmering water. Add the butter and chopped chocolate, stirring gently until completely melted and smooth. Remove from heat and let cool for about 5 minutes.
03 - Pour both 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.
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. Take care not to overmix, as this will affect the fudgy texture.
06 - If using, 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 it into an even layer. Smooth the surface with a spatula.
08 - Bake on the center rack for 22 to 25 minutes. The brownies are done when the top appears set and slightly crackled, but the center still has a gentle jiggle when lightly shaken.
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.