01 - Preheat oven to 350°F. Line a 9-inch square baking pan with parchment paper, leaving an overhang on two sides for easy removal.
02 - Set a heatproof bowl over a pot of barely simmering water. Add the chopped chocolate and butter, stirring occasionally 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 vigorously until well blended and no sugar lumps remain.
04 - Add the eggs one at a time, whisking thoroughly after each addition. Continue mixing until the batter becomes glossy, thick, and slightly aerated.
05 - Sift together 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, fold the chopped nuts and chocolate chips into the batter with a few gentle strokes to distribute evenly.
07 - Pour the batter into the prepared baking pan. Use an offset spatula to spread it evenly into the corners and smooth the surface.
08 - Bake on the center rack for 28 to 32 minutes, until the center is just set and a toothpick inserted comes out with a few moist crumbs clinging to it. Do not overbake.
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.