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 - In a heatproof bowl set over a pot of barely simmering water, melt the semisweet chocolate and butter together, stirring occasionally until completely smooth. Remove from heat and let the mixture cool for about 5 minutes.
03 - Add the granulated sugar and brown sugar to the melted chocolate mixture, whisking vigorously until well blended and slightly thickened.
04 - Whisk in the eggs one at a time, beating thoroughly after each addition until the batter turns glossy, thick, and 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 to avoid overmixing.
06 - If desired, fold in the chopped nuts and additional chocolate chips until evenly distributed throughout the batter.
07 - Pour the batter into the prepared baking pan and spread it into an even layer using a spatula, smoothing the surface.
08 - Bake for 28 to 32 minutes, or until the center is just barely set and a toothpick inserted into the middle emerges with a few moist crumbs clinging to it.
09 - Let the brownies cool completely in the pan on a wire rack. Once fully cooled, use the parchment overhang to lift the slab out and cut into 12 even squares.