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 pieces, stirring occasionally until completely melted and smooth. Remove from heat and let the mixture cool for about 5 minutes.
03 - Whisk the granulated sugar and brown sugar into the melted chocolate mixture until fully combined and no lumps remain.
04 - Add the eggs one at a time, whisking thoroughly after each addition. Continue whisking until the batter becomes 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. Be careful not to overmix, as this will produce a tougher crumb.
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 pan and use an offset spatula to spread it evenly, smoothing the surface.
08 - Bake on the center rack for 28 to 32 minutes, or until the edges pull away slightly from the pan and the center is just set. A toothpick inserted into the middle should come out with a few moist crumbs clinging to it.
09 - Let the brownies cool completely in the pan set on a wire rack. Once fully cooled, use the parchment overhang to lift the slab out of the pan and cut into 12 even squares.