01 - Preheat oven to 350°F (180°C). Line an 8×8-inch baking pan with parchment paper, leaving overhang on two sides for easy removal.
02 - Place butter and chopped dark chocolate in a heatproof bowl set over a saucepan of simmering water. Stir gently until completely melted and smooth. Remove from heat and let cool for 5 minutes.
03 - Add granulated sugar and light brown sugar to the melted chocolate mixture. Whisk until well blended and the mixture thickens slightly.
04 - Beat in eggs one at a time, whisking thoroughly after each addition until fully incorporated. Stir in vanilla extract until the batter is glossy and smooth.
05 - Sift flour, cocoa powder, and salt together into a separate bowl. Gradually fold the dry mixture into the wet batter using a spatula, mixing only until just combined. Avoid overmixing to keep brownies tender.
06 - Gently fold chopped nuts and/or chocolate chips into the batter if desired, distributing evenly throughout.
07 - Pour batter into the prepared baking pan and spread evenly. Smooth the surface with a spatula for a uniform bake.
08 - Bake for 22 to 25 minutes, or until the top is set and slightly cracked with a gentle jiggle remaining in the center. Do not overbake for fudgy results.
09 - Allow brownies to cool completely in the pan on a wire rack. Once fully set, lift out using parchment overhang and cut into 12 even squares.