INGREDIENTS
1 stick
unsalted butter, cut into large pieces, plus more for pan
3 tbsp
unbleached all-purpose flour, plus more for pan
6 oz
bittersweet chocolate (61 to 70 percent cacao), finely chopped
3/4 cup
plus 2 tablespoons sugar, divided
1
large egg yolk plus 3 large egg whites, room temperature, divided
1/4 cup
unsweetened cocoa powder, plus more for dusting
1/2 tsp
kosher salt
1/2 tsp
coarse salt