01 - Preheat the oven to 350°F. Grease a 9×9 inch square baking pan and line it with parchment paper, leaving overhang on two sides for easy removal.
02 - Place 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, combine the granulated sugar, brown sugar, eggs, and vanilla extract. Whisk vigorously 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 evenly incorporated.
05 - Sift the flour, cocoa powder, baking powder, and salt directly into the bowl. Using a spatula, gently fold the dry ingredients into the batter until just combined. Avoid overmixing to keep the brownies tender.
06 - If desired, fold in the chopped nuts and/or chocolate chips until evenly distributed throughout the batter.
07 - Pour the batter into the prepared baking pan and spread it into an even layer using the spatula, smoothing the surface.
08 - Bake on the center rack for 22–25 minutes, until the edges are set and the center still has a slight wobble. A toothpick inserted into the center should emerge with moist crumbs, not clean.
09 - Allow the brownies to cool completely in the pan on a wire rack before lifting them out with the parchment overhang and cutting into 12 even squares.