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Heat oven to 350º.
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Combine butter and sugar in large mixer bowl.
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Beat at medium speed, scraping bowl often, until creamy (1 to 2 minutes).
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Reduce speed to low and add flour and cornstarch; beat, scraping bowl often, until well-mixed (1 to 2 minutes).
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Stir in the chips by hand.
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Press into ungreased 13x9 pan.
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Prick all over with a fork.
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Bake 35 to 45 minutes or until edges just begin to brown.
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If browning too quickly, cover with aluminum foil.
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Cool in the pan for 15 minutes.
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Cut into bars while still warm.