01 - Preheat the oven to 350°F. Line an 8×8-inch baking pan with parchment paper, leaving enough overhang on two 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 fully incorporated and the mixture looks glossy.
04 - Add the eggs one at a time, whisking thoroughly after each addition until the batter is smooth and emulsified. Stir in the vanilla extract.
05 - Sift the all-purpose flour, cocoa powder, and salt directly into the batter. Using a spatula, fold the dry ingredients in gently until just combined. Be careful not to overmix, as this will make the brownies tough.
06 - If using, fold the chopped nuts and dark chocolate chips into the batter with a few light strokes.
07 - Pour the batter into the prepared baking pan and spread it into an even layer using a spatula. Smooth the surface to ensure uniform baking.
08 - Bake for 22 to 25 minutes. The brownies are done when the top is set and slightly crackly, but the center still has a gentle jiggle. 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 block out and slice into 12 even squares.