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 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 intervals. Let the mixture cool slightly.
03 - In a large mixing bowl, whisk together granulated sugar, brown sugar, eggs, and vanilla extract until the mixture turns pale and slightly thickened.
04 - Pour the melted chocolate mixture into the egg mixture and stir until well incorporated.
05 - Sift 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 desired, fold in chopped nuts and chocolate chips until evenly distributed throughout the batter.
07 - Pour the batter into the prepared baking pan and use a spatula to smooth the surface into an even layer.
08 - Bake for 22 to 25 minutes until edges are set but the center remains slightly fudgy. A toothpick inserted into the center should come out with moist crumbs clinging to it.
09 - Allow brownies to cool completely in the pan before slicing into squares.