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 up a double boiler by placing a heatproof bowl over a pot of barely simmering water. Add the chopped chocolate and butter pieces, stirring gently until completely melted and smooth. Remove from heat and let cool for 5 minutes.
03 - Add the granulated sugar and brown sugar to the melted chocolate mixture. Whisk vigorously until fully combined and no sugar granules remain visible.
04 - Crack in the eggs one at a time, whisking thoroughly after each addition. Continue whisking until the batter turns glossy, thick, and slightly aerated.
05 - Sift together the flour, cocoa powder, baking powder, and salt directly over the batter. Gently fold the dry mixture into the wet using a spatula, stopping as soon as no dry streaks remain. Take care not to overmix.
06 - If using chopped nuts or additional chocolate chips, fold them into the batter with a few gentle strokes to distribute evenly.
07 - Pour the batter into the prepared pan and spread it into an even layer using a spatula. Smooth the surface to ensure uniform thickness.
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 - Let the brownies cool completely in the pan set on a wire rack. Once fully cooled, grasp the parchment overhang to lift the slab out, then cut into 12 even squares.