01 - Preheat oven to 350°F. Line an 8×8-inch baking pan with parchment paper, leaving overhang on two sides for easy removal.
02 - Set a heatproof bowl over a saucepan of barely simmering water. Add butter and chopped chocolate, stirring occasionally until completely melted and smooth. Remove from heat and let cool slightly for about 5 minutes.
03 - Pour both granulated and light brown sugars into the melted chocolate mixture. Whisk vigorously until fully combined and smooth.
04 - Add eggs one at a time, whisking thoroughly after each addition until the batter is glossy and well emulsified. Stir in vanilla extract until evenly distributed.
05 - Sift flour, cocoa powder, and salt together directly into the batter. Using a spatula, gently fold the dry ingredients into the wet mixture until just combined. Avoid overmixing to maintain a tender crumb.
06 - If desired, gently fold chopped nuts and/or chocolate chips into the batter with a few light strokes.
07 - Pour the batter into the prepared baking pan and spread evenly. Use a spatula to smooth the surface.
08 - Bake for 22–25 minutes, or until the edges are set and the top has formed a thin crackly crust. The center should have a slight jiggle when gently shaken — it will firm up as it cools.
09 - Allow brownies to cool completely in the pan on a wire rack. Once fully cooled, lift out using parchment overhang and slice into 12 even squares.