01 - Preheat oven to 350°F. Line a 9-inch square baking pan with parchment paper and lightly grease it.
02 - Melt dark chocolate and butter together in a heatproof bowl over simmering water, stirring until smooth. Remove from heat and let cool slightly.
03 - Whisk eggs, granulated sugar, and brown sugar in a large bowl until pale and thick, about 2 minutes.
04 - Stir melted chocolate mixture into the egg mixture until fully incorporated.
05 - Sift flour, cocoa powder, and salt into the mixture. Gently fold until just combined to avoid overmixing.
06 - Fold in chopped nuts and/or chocolate chips if desired.
07 - Pour batter into prepared pan and smooth the surface evenly.
08 - Bake 28 to 32 minutes, or until the top is set and a toothpick inserted in the center comes out with moist crumbs.
09 - Allow brownies to cool completely in the pan before removing and slicing into squares.