Preheat the oven to 350°F and line a 9×13-inch pan with foil or parchment, leaving an overhang for easy removal; set a rack in the middle position.

Place chopped semisweet chocolate and butter in a heatproof bowl over simmering water and stir until just melted and smooth, then remove from heat and cool for 3–4 minutes until warm but not hot.

Whisk the eggs, egg yolks, granulated sugar, and light brown sugar in a large bowl until thick and slightly ribboning (2–3 minutes with a mixer or 4–5 minutes by hand).

Slowly drizzle one-third of the warm chocolate into the egg mixture while whisking constantly to temper the eggs; fold in remaining chocolate with a spatula, then add cocoa, salt, vanilla, and fold in flour until just combined.

Fold in chopped chocolate chunks and nuts if using, taking care not to overmix.

Spread batter evenly in the prepared pan and bake at 350°F for 20–24 minutes; start checking at 18 minutes—edges should be set, center should jiggle slightly and a toothpick should come out with moist crumbs.

Remove from oven and let the pan rest on a wire rack for 25–30 minutes to finish cooking gently and develop a crackly top.

After the warm rest, lift the brownies from the pan using the overhang and transfer to a cutting board; for cleaner slices chill for 15–20 minutes before cutting into 16 squares and garnish with sprinkles or berries if desired.