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 dark chocolate, stirring gently until completely melted and smooth. Remove from heat and let cool for about 5 minutes.
03 - Add the granulated sugar and light brown sugar to the chocolate mixture. Whisk until fully combined and the mixture appears glossy.
04 - Crack in the eggs one at a time, whisking thoroughly after each addition until the batter is smooth and well emulsified. Stir in the vanilla extract.
05 - Sift the flour, cocoa powder, and salt together directly into the batter. Using a spatula, gently fold the dry ingredients into the wet mixture until just combined. Be careful not to overmix.
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 use a spatula to smooth the surface into an even layer.
08 - Bake for 22 to 25 minutes, or until the top is set and slightly crackled with a gentle jiggle remaining in the center. Avoid overbaking for the best fudgy texture.
09 - Allow the brownies to cool completely in the pan on a wire rack. Once fully cooled, lift them out using the parchment overhang and slice into 12 even squares.