01 - Preheat the 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 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 both the granulated and brown sugars into the melted chocolate mixture. Whisk vigorously until fully blended and no sugar granules remain on the surface.
04 - Add the eggs one at a time, whisking thoroughly after each addition. Continue whisking until the batter turns glossy, thick, and slightly aerated.
05 - Sift the flour, cocoa powder, baking powder, and salt directly into the batter. Using a spatula, gently fold the dry ingredients into the chocolate mixture until just incorporated. Stop as soon as no dry streaks remain to keep the texture tender.
06 - If desired, fold in the chopped nuts and chocolate chips until evenly distributed throughout the batter.
07 - Pour the batter into the prepared pan and spread it into an even layer. Use a spatula to smooth the surface.
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 - Remove from the oven and place the pan on a wire rack. Allow the brownies to cool completely in the pan before using the parchment overhang to lift them out. Cut into 12 even squares with a sharp knife.