Preheat oven 350°F, line 9×9 pan.
Melt butter, chocolate, cocoa, vanilla → smooth. Cool slightly.
Beat eggs + sugars 7–10 min. Add discard + chocolate, mix. Stir in flour + salt (batter thick).
Spread in pan, bake 35–40 min. Cool fully before cutting.
Storage: Airtight up to 7 days, freeze up to 3 months.