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 simmering water, stirring until smooth. Alternatively, microwave in 30-second bursts, stirring between each interval. Let the mixture cool slightly.
03 - In a large mixing bowl, whisk together the granulated sugar, brown sugar, eggs, and vanilla extract until the mixture turns pale and becomes slightly thickened, about 2 minutes.
04 - Pour the melted chocolate mixture into the egg mixture and stir until fully incorporated and homogeneous.
05 - Sift the flour, cocoa powder, baking powder, and salt directly into the batter. Gently fold with a spatula until just combined, being careful not to overmix.
06 - If using, fold the chopped nuts and/or chocolate chips into the batter with a few gentle strokes.
07 - Pour the batter into the prepared baking pan and use the spatula to smooth the surface into an even layer.
08 - Bake for 22 to 25 minutes, until the edges are firm and the center remains slightly fudgy. A toothpick inserted into the center should emerge with moist crumbs attached.
09 - Allow the brownies to cool completely in the pan before cutting into 12 even squares.