01 - Preheat oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
02 - In a microwave-safe bowl, melt butter and chopped chocolate together in 20-second intervals, stirring between each, until smooth.
03 - Whisk in sugar until combined and slightly cooled.
04 - Add eggs and vanilla extract. Whisk vigorously for 1–2 minutes until the mixture is shiny and thick.
05 - Sift in the flour, cocoa powder, and salt. Gently fold with a spatula until just combined — do not overmix.
06 - Fold in chocolate chips or nuts if using.
07 - Pour the batter into the prepared pan and spread evenly.
08 - Bake for 25–30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs (not wet batter).
09 - Let cool completely in the pan before slicing into squares.