01 - Preheat the oven to 350°F. Grease a 9×9-inch square baking pan and line it with parchment paper, leaving enough overhang on the sides to lift the brownies out later.
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, whisk together the granulated sugar, brown sugar, eggs, and vanilla extract vigorously until the mixture turns pale in color and becomes slightly thickened, about 2 minutes.
04 - Pour the melted chocolate mixture into the egg mixture and stir gently 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 wet until just combined. Be careful not to overmix, as this will make the brownies tough.
06 - If using, 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 use the spatula to smooth the top into an even layer. Bake for 22 to 25 minutes, until the edges are fully set but the center still has a slight wobble. A toothpick inserted into the center should come out with moist crumbs clinging to it.
08 - Allow the brownies to cool completely in the pan on a wire rack before lifting them out using the parchment overhang and slicing into 12 even squares.