01 - Preheat oven to 350°F. Grease a 9x9 inch square baking pan and line with parchment paper, leaving overhang on two sides for easy removal.
02 - Combine the chopped dark chocolate and butter in a heatproof bowl set over a pot of barely simmering water, stirring occasionally until completely smooth. Alternatively, melt in the microwave in 30-second bursts, stirring between intervals. Let the mixture cool slightly.
03 - In a large mixing bowl, vigorously whisk together the granulated sugar, brown sugar, eggs, and vanilla extract until the mixture turns pale yellow and becomes slightly thickened, about 2 minutes.
04 - Pour the melted chocolate mixture into the egg mixture and stir with a spatula until fully incorporated and homogeneous.
05 - Sift the flour, cocoa powder, baking powder, and salt directly into the wet mixture. Using a spatula, gently fold the dry ingredients into the batter until just combined. Take care not to overmix, as this will toughen the crumb.
06 - If desired, fold in the chopped nuts and/or chocolate chips until evenly distributed throughout the batter.
07 - Transfer the batter into the prepared baking pan and use the spatula to spread it evenly, smoothing the surface.
08 - Bake for 22 to 25 minutes, or until the edges are set and pulling away slightly from the pan while the center still appears slightly soft and fudgy. A toothpick inserted into the center should come out with moist crumbs clinging to it, not clean.
09 - Allow the brownies to cool completely in the pan on a wire rack before cutting into squares. This resting period helps the texture set properly for cleaner cuts.