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 about 5 minutes.
03 - Pour the granulated sugar and brown sugar into the melted chocolate mixture. Whisk thoroughly until both sugars are fully blended and no gritty pockets remain.
04 - Crack in the eggs one at a time, whisking vigorously after each addition. Continue whisking until the batter turns glossy, thick, and slightly elastic, about 1 to 2 minutes.
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 to keep the brownies tender.
06 - If using, fold the chopped nuts and chocolate chips into the batter with a few light strokes to distribute evenly without deflating the mixture.
07 - Scrape the batter into the prepared baking pan. Use an offset spatula to spread it into an even layer, pushing it into the corners.
08 - Place the pan on the center rack and bake for 28 to 32 minutes. Test 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, use the parchment overhang to lift the slab out and cut into 12 even squares.