Prep the pan and oven. Heat oven to 350°F (175°C). Line an 8-inch square pan with parchment, leaving overhang for easy lifting. Lightly grease the parchment.

Brown the butter. In a light-colored saucepan, melt the butter over medium heat. Cook, stirring, until it foams and the milk solids turn amber and smell nutty, 4–6 minutes. Remove from heat.

Melt with chocolate. Immediately add the chopped dark chocolate to the hot browned butter. Let sit 1 minute, then stir until smooth and glossy.

Whisk eggs and sugars. In a mixing bowl, whisk granulated sugar, brown sugar, eggs, and egg yolk for 1–2 minutes until slightly thick and lighter. This helps create that shiny top.

Combine. Whisk in vanilla and the warm chocolate-butter mixture until fully blended.

Fold in dry ingredients. Sift in flour, cocoa, and salt. Gently fold with a spatula just until no dry streaks remain. Don’t overmix. Fold in any mix-ins.

Pan and bake. Spread the batter into the pan, smoothing the top. Bake 22–28 minutes. The edges will be set, the center slightly soft, and a toothpick should come out with moist crumbs, not wet batter.

Finish and cool. If using, sprinkle flaky sea salt on top. Cool in the pan for at least 1 hour, then lift out and cut with a warm, clean knife for sharp edges.