Preheat the oven to 175°C (350°F). Line an 8x8-inch (20x20 cm) square baking pan with parchment paper, leaving an overhang for easy removal. Lightly butter the parchment or spray with nonstick spray and dust with a little cocoa powder, tapping out excess.

Measure all ingredients so they are ready. Chop the chocolate and nuts (if using) so they are uniform in size.

Making the batter

Melt butter and chocolate

Place the butter and chopped chocolate in a heatproof bowl set over a pot of barely simmering water (double boiler) or melt gently in short bursts in the microwave, stirring every 20–30 seconds until smooth. Remove from heat and let cool for 3–5 minutes.

Combine sugar and chocolate

Stir the granulated sugar into the slightly cooled melted chocolate until evenly combined. The mixture will feel grainy at first — that’s normal.

Add eggs and vanilla

Whisk in the eggs one at a time until fully incorporated and the batter is glossy. Stir in the vanilla extract.

Dry ingredients

In a separate bowl, whisk together the all-purpose flour, unsweetened cocoa powder, fine salt, and espresso powder (if using) to break up lumps.

Combine wet and dry

Fold the dry ingredients into the chocolate mixture with a rubber spatula until just combined and no streaks of flour remain. Do not overmix. If using, fold in the chopped nuts and/or chocolate chips, reserving a small handful to sprinkle on top.

Baking and finishing

Bake

Scrape the batter into the prepared pan and spread evenly with an offset spatula. Sprinkle the reserved chips/nuts on top and lightly press so they adhere. Bake in the preheated oven for 22–28 minutes, rotating the pan halfway through. The brownies are done when the edges are set and a toothpick inserted near the center comes out with a few moist crumbs but not wet batter.

Cool

Remove from oven and allow the brownies to cool in the pan on a wire rack for at least 30 minutes. Use the parchment overhang to lift the brownie slab out of the pan and cool completely on the rack for cleaner slices. If desired, sprinkle with a pinch of coarse sea salt while still slightly warm.

Slice and serve

Once cooled, slice into 9 or 12 squares using a sharp knife wiped clean between cuts for neat edges. Serve as-is or with a scoop of vanilla ice cream or a dusting of powdered sugar.