INGREDIENTS
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1 pound bittersweet chocolate
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12 ounces semisweet chocolate
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3/4 cup egg whites (4 to 5 extra-large eggs), at room temperature
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1 1/2 cups granulated sugar
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1/4 teaspoon cream of tartar
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3/4 teaspoon kosher salt
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1 1/2 pounds unsalted butter, at room temperature
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3 teaspoons pure vanilla extract
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3 teaspoons instant espresso powder, dissolved in 1 1/2 teaspoons water
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3 tablespoons dark rum, optional