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Position rack in upper third of oven and preheat to 325 degrees.
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Lightly oil 2 nonstick cookie sheets.
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Beat whites at medium speed until foamy.
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Add cream of tartar and continue beating until soft peaks form.
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Add 1/4 cup sugar ,1 tablespoon at a time, and beat until whites are stiff and shiny but not dry.
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Sift cocoa, cornstarch, and remaining 1/4 cup sugar together.
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Sprinkle half of mixture over whites;using rubber spatula, gently fold in just until blended.
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Repeat,folding in remaining cocoa mixture, then pecans.
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Spoon mixture into pastry bag fitted with 1/2-inch star tip.
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Pipe mixture onto prepared sheets into 1-to 1 1/2-inch rounds (or drop by teaspoonfuls).
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Bake cookies until set, about 35 minutes.
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Cool slightly.
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Using spatula, gently transfer cookies to racks to cool completely.
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Can be prepared 1 day ahead. Store airtight at room temperature.