INGREDIENTS
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Unsalted butter, room temperature, for baking dish
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1/4 cup sugar, plus more for baking dish
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8 ounces semisweet chocolate, finely chopped, or semisweet chocolate chips (1 cup)
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1 teaspoon pure vanilla extract
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3 large egg yolks, lightly beaten, plus 4 large egg whites
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1/4 teaspoon cream of tartar