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