01 - Preheat oven to 350°F. Line a 9-inch square baking pan with parchment paper, leaving an overhang on two opposite sides for easy removal.
02 - Set a heatproof bowl over a pot of barely simmering water. Add the chopped chocolate and butter, stirring gently until completely melted and smooth. Remove from heat and let cool for 5 minutes.
03 - Pour the granulated sugar and brown sugar into the melted chocolate mixture. Whisk vigorously until both sugars are fully dissolved and the mixture appears uniform.
04 - Crack in the eggs one at a time, whisking thoroughly after each addition. Continue whisking until the batter becomes glossy, thick, and slightly aerated.
05 - Sift the flour, cocoa powder, baking powder, and salt directly into the batter. Using a spatula, fold the dry ingredients in gently until just incorporated — a few streaks of flour are fine. Do not overmix.
06 - If using, fold in the chopped nuts and 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 an offset spatula, ensuring it reaches all corners.
08 - Bake on the center rack for 28 to 32 minutes. The brownies are done when the edges have pulled away slightly, the center is just set, and a toothpick inserted in the middle comes out with a few moist crumbs clinging to it.
09 - Let the brownies cool completely in the pan set on a wire rack. Once fully cooled, use the parchment overhang to lift the slab out and cut into 12 even squares.