01 - Preheat oven to 350°F. Line a 9-inch square baking pan with parchment paper, leaving an overhang on two opposing 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 fully blended and no sugar granules remain visible.
04 - Add eggs one at a time, whisking thoroughly after each addition. Continue whisking until the batter turns glossy, thick, and well emulsified.
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. Stop mixing as soon as no dry streaks remain.
06 - If desired, fold in the chopped walnuts, pecans, or chocolate chips until evenly distributed throughout the batter.
07 - Pour the batter into the prepared baking pan. Use an offset spatula to spread it evenly and smooth the surface.
08 - Bake on the center rack for 28 to 32 minutes. Test for doneness by inserting a toothpick into the center—it should come out with a few moist crumbs clinging to it, not wet batter.
09 - Transfer the pan to a wire rack and let the brownies cool completely. Once fully set, lift them out using the parchment overhang and cut into 12 even squares.