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