01 - Preheat oven to 350°F. Line a 9-inch square baking pan with parchment paper, leaving an overhang on two opposite sides for easy removal.
02 - Set a heatproof bowl over a pot of barely simmering water. Add the chopped chocolate and butter, stirring occasionally until completely melted and smooth. Remove from heat and let cool for 5 minutes.
03 - Whisk the granulated sugar and brown sugar into the melted chocolate mixture until fully dissolved and well combined.
04 - Add eggs one at a time, whisking thoroughly after each addition. Continue whisking until the batter becomes glossy, thick, and smooth.
05 - Sift together the flour, cocoa powder, baking powder, and salt into a separate bowl. Gently fold the dry mixture into the chocolate batter using a spatula, mixing until just incorporated. Do not overmix.
06 - If desired, gently fold chopped nuts or additional chocolate chips into the batter until evenly distributed.
07 - Pour the batter into the prepared pan and spread evenly. Smooth the surface with a spatula to ensure uniform thickness.
08 - Bake for 28 to 32 minutes, or until the center is just set and a toothpick inserted comes out with a few moist crumbs clinging to it.
09 - Allow the brownies to cool completely in the pan on a wire rack. Once fully cooled, use the parchment overhang to lift the block out and cut into 12 even squares.