Preheat the oven to 175°C (350°F). Line an 8-inch (20 cm) square baking pan with parchment paper leaving a 2-inch overhang on two opposite sides for easy removal. Lightly grease the paper.

Melt butter and chocolate

Chop the semi-sweet chocolate into small pieces. In a heatproof bowl set over a pan of barely simmering water (double boiler) or in short bursts in the microwave (30 seconds, stir, repeat), melt together the butter and chocolate until smooth. Remove from heat and let cool for 3–5 minutes.

Combine sugar and eggs

Place the granulated sugar in a mixing bowl. Add the eggs one at a time, whisking vigorously after each addition until fully incorporated and the mixture is glossy. Whisk in the vanilla extract.

Temper chocolate into eggs

Slowly pour about one-third of the warm chocolate-butter mixture into the egg-sugar mixture while whisking constantly to temper the eggs. Then pour in the remaining chocolate and fold gently with a spatula until uniform.

Add dry ingredients

Sift together the all-purpose flour, unsweetened cocoa powder, fine salt, and optional espresso powder into the chocolate-egg mixture. Fold with a spatula just until no streaks of flour remain; do not overmix.

Fold in mix-ins

Fold in the chopped nuts and dark chocolate chips, reserving a small handful to sprinkle on top if desired.

Bake

Scrape the batter into the prepared pan and smooth the top. Sprinkle reserved chips and a light pinch of flaky sea salt over the surface. Bake in the preheated oven for 22–28 minutes, or until the edges are set and a toothpick inserted into the center comes out with a few moist crumbs (not wet batter).

Cool

Remove from oven and set the pan on a wire rack. Let the brownies cool completely in the pan (about 1–2 hours) so they firm up for clean slicing. Use the parchment overhang to lift the slab from the pan.

Optional finishing

Slice: Using a sharp knife, trim the edges if desired and cut into 9 or 12 squares. For clean cuts, chill the baked brownies for 30 minutes and wipe the knife clean between cuts.

Storage: Store cooled brownies in an airtight container at room temperature up to 3 days or refrigerated up to 7 days. For longer storage, freeze individual squares wrapped tightly for up to 2 months.