"These rich treats aren't cookie-jar cookies—they're best the day they're made...."
INGREDIENTS
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Nonstick vegetable oil spray
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1 cup bittersweet chocolate chips (do not exceed 61% cacao; about 6 ounces), divided
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1 1/2 cups powdered sugar, divided
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1/3 cup unsweetened cocoa powder
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2 teaspoons cornstarch
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2 large egg whites, room temperature
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1/2 teaspoon vanilla extract
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1/8 teaspoon cream of tartar