01 - Preheat the 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 pieces, stirring gently until completely melted and smooth. Remove from heat and let the mixture cool for about 5 minutes.
03 - Pour the granulated sugar and brown sugar into the melted chocolate mixture. Whisk thoroughly until both sugars are fully dissolved and evenly distributed.
04 - Crack in the eggs one at a time, whisking vigorously after each addition. Continue whisking until the batter turns glossy, thick, and slightly increased in volume.
05 - Sift the flour, cocoa powder, baking powder, and salt directly over the batter. Using a spatula, fold the dry ingredients in gently until just incorporated. Avoid overmixing to keep the brownies tender.
06 - If using chopped nuts or additional chocolate chips, fold them into the batter now with a few gentle strokes to distribute evenly.
07 - Pour the batter into the prepared pan and spread it into an even layer using an offset spatula, working into the corners.
08 - Place the pan on the center rack and bake for 28 to 32 minutes. The brownies are done when the center is just set and a toothpick inserted comes out with a few moist crumbs clinging to it.
09 - Let the brownies cool completely in the pan on a wire rack. Once fully cooled, lift them out using the parchment overhang and cut into 12 even squares with a sharp knife.