01 - Preheat the oven to 350°F (180°C). Line an 8x8-inch baking pan with parchment paper, leaving an 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 occasionally until completely smooth. Remove from heat and let the mixture cool slightly for about 5 minutes.
03 - Add both the granulated sugar and light brown sugar to the chocolate mixture. Whisk thoroughly until well combined and the mixture appears glossy.
04 - Add the eggs one at a time, whisking vigorously after each addition until fully incorporated. Stir in the vanilla extract until the batter is smooth and uniform.
05 - Sift the all-purpose flour, unsweetened cocoa powder, and fine salt together directly into the batter. Gently fold the dry ingredients into the wet mixture using a spatula until just combined. Take care not to overmix, as this can produce a tougher crumb.
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 use a spatula to spread it into an even layer, smoothing the surface.
08 - Bake for 22 to 25 minutes, or until the top is set and slightly crackly while the center still has a gentle jiggle. Avoid overbaking for the best fudgy texture.
09 - Allow the brownies to cool completely in the pan on a wire rack before using the parchment overhang to lift them out. Slice into 12 even squares.