Preheat oven to 350°F (175°C). Grease or line your 13×9 pan with parchment.

Melt butter, then whisk in sugar until combined.

Add eggs + extra yolk + vanilla, whisk until glossy and slightly thick.

Mix in cocoa powder, salt, baking powder.

Fold in flour just until no dry streaks remain.

Spread evenly into pan.

Bake 25–32 minutes (start checking at 25 min, toothpick should come out with moist crumbs, not clean).

Cool before slicing (40 minutes on countertop, then preferably 1 hour in the fridge).