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Preheat oven to 325°F.
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Line 8" square baking pan with enough foil to overhand sides by 2"; coat foil with non-stick cooking spray.
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On med-high speed, beat 1/2 cup powdered sugar, cream cheese and butter until blended. Add flour and salt; beat on low speed until dough just forms. Press dough into bottom and 1/2" up sides of pan; prick with fork. Bake 20-25 minutes or until just golden.
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Meanwhile, in bowl, whisk together eggs, egg whites, lemon juice, baking powder and remaining 1 cup powdered sugar until combined. Stir in lemon zest. Pour over warm crust.
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Bake 25 minutes or until filling is set.
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Cool completely in pan on rack.
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Cover; refrigerate until chilled, at least 1 hour. Using foil to help, lift from pan. Cut into 16 bars. If desired, sprinkle with additional powdered sugar.