Preheat the oven to 180°C (350°F). Line a 20 x 20 cm (8 x 8 in) square baking pan with parchment paper, leaving a 2–3 cm overhang on two opposite sides to lift brownies out easily. Lightly butter or spray the parchment.

If using nuts, toast them in a dry skillet over medium heat for 4–5 minutes, stirring, until fragrant and lightly browned. Let cool and roughly chop. Measure and set aside.

Make the Batter

Melt butter and chocolate

Place the chopped chocolate and butter in a heatproof bowl set over a saucepan of gently simmering water (double boiler), making sure the bowl does not touch the water. Stir occasionally until smooth and fully melted. Remove from heat and let cool 3–5 minutes.

Combine sugar and eggs

Whisk the granulated sugar into the slightly cooled chocolate mixture until combined. Add the eggs one at a time, whisking briskly after each addition until the mixture is glossy. Whisk in the vanilla extract.

Fold dry ingredients

Sift together the flour, cocoa powder, and salt into a bowl. Fold the dry ingredients into the chocolate mixture with a flexible spatula in two additions, mixing just until no streaks of flour remain. Avoid overmixing to keep brownies fudgy.

Add nuts (optional)

Fold in the toasted chopped nuts if using, reserving a small sprinkle for the top if desired.

Bake & Finish

Bake

Spread the batter evenly into the prepared pan, smoothing the top with an offset spatula. If saving some nuts for the top, sprinkle them evenly now. Bake in the preheated oven for 20–25 minutes. Brownies are done when the edges are set, the center is slightly jiggly, and a toothpick inserted in the center comes out with a few moist crumbs (not wet batter).

Cool

Remove the pan from the oven and set on a wire rack. Let cool completely in the pan (about 1–1.5 hours) so the brownies set; this makes clean squares. For faster handling, chill in the refrigerator for 30–45 minutes after they reach room temperature.

Cut for to-go

Use the parchment overhang to lift the brownie slab from the pan. Place on a cutting board and, using a sharp knife wiped between cuts, cut into 6 equal squares (for larger, fudgy to-go portions) or 9 smaller squares if desired. Optionally dust the tops with powdered sugar and finish with a light sprinkle of flaky sea salt.

Package

Place each brownie square on a parchment paper square or liner and transfer into individual kraft boxes or reusable containers. Seal and label with stickers or tape. If giving warm, allow to cool slightly so packaging won't trap steam. Store at room temperature up to 3 days or refrigerate up to 7 days. For best texture, bring to room temperature before serving.