Step 1: Preheat the oven to 350°F (175°C). Line an 8 × 8-inch (20 × 20 cm) baking pan with parchment paper and lightly grease the exposed sides.

Step 2: Melt the chopped chocolate and butter in a heatproof bowl over gently simmering water or in the microwave in 20-second bursts. Stir until smooth.

Step 3: Let the mixture cool for 3–5 minutes, then whisk in the sugar until combined.

Step 4: Whisk in the eggs one at a time, followed by the vanilla. Continue whisking for about one minute until thick and glossy.

Step 5: Sift in the cocoa powder and salt. Fold gently until no dry streaks remain. Stir in optional chocolate chips or walnuts.

Step 6: Pour the batter into the prepared pan and smooth the surface. Bake for 20–22 minutes, until the edges are set and a toothpick inserted in the center shows a few moist crumbs.

Step 7: Cool in the pan for at least 30 minutes. Lift out using the parchment paper and cut into 16 squares with a sharp knife.