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 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 vigorously until fully blended and no sugar granules remain visible.
04 - Add 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. Gently fold the dry ingredients into the chocolate mixture using a spatula, stopping as soon as no dry streaks remain. Overmixing will produce a dense, tough crumb.
06 - If using chopped nuts or additional chocolate chips, fold them in gently with a spatula until evenly distributed throughout the batter.
07 - Pour the batter into the prepared pan and spread it into an even layer. Smooth the surface with a spatula to ensure uniform thickness across the pan.
08 - Bake on the center rack 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 - Let the brownies cool completely in the pan set on a wire rack. Once fully cooled, lift the brownies out using the parchment overhang and cut into 12 even squares with a sharp knife.