01 - Preheat oven to 350°F. Line a 9-inch square baking pan with parchment paper, allowing overhang for easy removal.
02 - Set a heatproof bowl over a saucepan of gently simmering water. Melt chocolate and butter together, stirring until smooth. Remove from heat and cool slightly.
03 - Whisk granulated and brown sugars into the melted chocolate mixture until thoroughly combined.
04 - Add eggs one at a time, whisking well after each addition until the mixture appears glossy and smooth.
05 - Sift flour, cocoa powder, and salt into the bowl. Gently fold with a spatula just until no flour streaks remain.
06 - Fold in nuts or chocolate chips if desired.
07 - Pour batter into the prepared pan and smooth the top surface evenly.
08 - Bake for 28 to 32 minutes until the top is set and a toothpick inserted in the center emerges with a few moist crumbs.
09 - Cool completely in the pan. Lift out using parchment overhang and cut into squares.