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

Melt chocolate and butter: In a heatproof bowl, melt butter and chopped dark chocolate together over a gentle simmer (double boiler) or in short microwave bursts. Stir until glossy and smooth. Let cool 3–4 minutes.

Whisk in sugars: Add granulated and brown sugar to the warm chocolate mixture. Whisk until combined and slightly thick. This helps create that shiny top.

Add eggs and vanilla: Whisk in eggs one at a time, mixing well after each. Add vanilla. The batter should look silky and a bit elastic.

Sift dry ingredients: In a small bowl, sift flour, cocoa powder, and salt. This prevents dry clumps.

Fold the batter: Use a spatula to gently fold dry ingredients into the wet mixture just until combined. Do not overmix. If using mix-ins, fold them in now.

Pan and smooth: Spread batter into the prepared pan, smoothing the top. For extra flair, sprinkle a pinch of flaky salt.

Bake: Bake 20–24 minutes. The edges should be set, and a toothpick inserted 2 inches from the edge comes out with a few moist crumbs. The center should look slightly underdone.

Cool completely: Let the brownies cool in the pan on a rack for at least 1 hour. For the cleanest cuts and fudgiest texture, chill 30 minutes before slicing.

Slice: Lift out using the parchment and cut into 9 or 16 squares with a sharp knife, wiping the blade between cuts.