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 up a double boiler by placing a heatproof bowl over a pot of barely simmering water. Add the chopped chocolate and butter pieces, stirring gently until completely melted and smooth. Remove from heat and allow to cool for 5 minutes.
03 - Pour the granulated sugar and brown sugar into the melted chocolate mixture. Whisk vigorously until both sugars are fully dissolved and the mixture appears glossy.
04 - Add the eggs one at a time, whisking thoroughly after each addition. Continue mixing until the batter becomes thick, shiny, and falls in smooth ribbons from the whisk.
05 - Sift the flour, cocoa powder, baking powder, and salt directly into the chocolate batter. Using a spatula, fold the dry ingredients in gently until just incorporated — a few streaks of flour are fine. Avoid overmixing to keep the brownies tender.
06 - If desired, 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 pan and spread it into an even layer using an offset spatula, working into the corners.
08 - Bake for 28 to 32 minutes, rotating the pan halfway through. The brownies are done when the center is just set and a toothpick inserted comes out with a few moist crumbs clinging to it.
09 - Transfer the pan to a wire rack and let the brownies cool completely undisturbed. Once fully cooled, use the parchment overhang to lift the slab out and cut into 12 even squares.