Fudgy Brownie Cake

Ingredients

• 1 stick unsalted butter, cut into large pieces, plus more for pan
• 3 tablespoons all-purpose flour, plus more for pan
• 6 ounces 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 teaspoon coarse salt