Preheat oven to 175°C (350°F). Line a 9x13-inch (23x33 cm) baking pan with parchment paper leaving an overhang on two sides for easy removal; lightly butter the parchment.

Melt chocolate and butter

Chop the semisweet chocolate into small pieces. Combine chocolate and butter in a heatproof bowl set over a barely simmering pot of water (double boiler) or microwave in 20–30 second bursts, stirring between bursts, until fully melted and smooth. Remove from heat and let cool 3–5 minutes.

Mix sugar and eggs

In a large bowl, whisk together granulated sugar and eggs until combined and slightly thickened (about 30–60 seconds by hand or 1 minute with a whisk/hand mixer on low).

Combine chocolate with eggs

Slowly pour the slightly cooled melted chocolate-butter mixture into the egg-sugar mixture while whisking constantly to temper and prevent cooking the eggs. Stir in the vanilla extract.

Sift dry ingredients

Sift together the all-purpose flour, unsweetened cocoa powder, fine salt, and baking powder into a separate bowl to remove lumps and ensure even distribution.

Fold dry into wet

Using a rubber spatula, fold the sifted dry ingredients into the chocolate mixture in two additions. Mix just until you no longer see streaks of flour—do not overmix to keep fudgy texture. If using nuts, fold them in gently at the end.

Transfer to pan and bake

Scrape the batter into the prepared pan and smooth the top with an offset spatula. Tap the pan once on the counter to release large air bubbles. Bake in the preheated oven for 25–30 minutes for fudgy brownies (a toothpick inserted in the center should come out with a few moist crumbs). For slightly more cake-like brownies, bake 30–35 minutes.

Cool

Remove from oven and place the pan on a wire rack. If using sea salt, sprinkle it evenly over the warm brownie surface. Let cool completely in the pan (at least 1 hour) before lifting out with the parchment overhang. Cooling fully allows cleaner slices.

Finishing

Once brownies are completely cool, optionally dust the top lightly with powdered sugar using a fine sieve. Use a sharp knife warmed under hot water and wiped dry between passes to cut neat squares (recommend 12 large or 24 smaller pieces). Serve at room temperature.