INGREDIENTS
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10 tablespoons plus 1 tablespoon unsalted butter, room temperature, divided
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2/3 cup unsweetened cocoa powder, plus 1 tablespoon for dusting
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10 1/2 ounces bittersweet or semisweet chocolate (50 to 64 percent cacao), finely chopped
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5 large eggs, separated (see Recipe Notes)
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1/2 teaspoon espresso powder
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1/4 teaspoon salt
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1 teaspoon pure vanilla extract
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1/2 teaspoon cream of tartar
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1 cup granulated sugar