Preheat the oven to 175°C (350°F). Lightly grease a 23 x 23 cm (9 x 9 in) square baking pan and line it with parchment paper leaving an overhang on two opposite sides to lift brownies out easily.

Measure and assemble all ingredients. If using nuts, toast them in a dry skillet over medium heat until fragrant, about 4–5 minutes, stirring; cool and coarsely chop.

Melt Chocolate and Butter

Double-boiler method

Place the chopped bittersweet chocolate and butter in a heatproof bowl set over a simmering pot of water (bottom of bowl should not touch the water). Stir occasionally until both are melted and smooth, 4–6 minutes. Remove from heat and let cool slightly (3–5 minutes).

Microwave method (alternative)

Combine butter and chopped chocolate in a microwave-safe bowl. Microwave in 20–30 second bursts, stirring between each, until melted and smooth. Let cool slightly.

Make the Batter

Whisk the granulated sugar into the warm melted chocolate-butter mixture until fully combined and slightly glossy.

Add the eggs one at a time, whisking vigorously after each addition until the mixture is smooth and slightly thickened. Stir in the vanilla extract.

Sift the all-purpose flour and unsweetened cocoa powder together over the chocolate mixture to avoid lumps. Add the fine salt. Fold everything together gently with a spatula until just combined — do not overmix. The batter should be thick and fudgy.

Fold in the chopped toasted nuts and/or chocolate chips if using, reserving a few to scatter on top for presentation.

Bake

Scrape the batter into the prepared pan and smooth the top. Sprinkle the reserved chocolate pieces and a light pinch of flaky sea salt over the surface if desired.

Bake in the preheated oven for 22–28 minutes. Start checking at 20 minutes: the edges should be set and a thin crust should form on top, while a toothpick inserted into the center will come out with a few moist crumbs (not wet batter) for a fudgy center.

Remove from oven and let the brownies cool completely in the pan on a wire rack (at least 1–1.5 hours) to allow them to set. For cleaner cuts, chill in the refrigerator for 30–60 minutes after they reach room temperature.

Serve & Store

Use the parchment overhang to lift the brownie slab from the pan. Cut into 9 or 12 squares with a sharp knife, wiping the knife between cuts for neat slices.

Store leftover brownies in an airtight container at room temperature for up to 3 days, or refrigerate for up to 7 days. Bring to room temperature or warm briefly before serving.