1Heat oven to 325°F (if using dark or nonstick pan, heat oven to 300°F). Grease bottom and sides of 13x9-inch pan with shortening or spray with cooking spray.
2In large bowl, beat 3/4 cup powdered sugar, the granulated sugar and butter with electric mixer on low speed until combined. Beat on medium speed until light and fluffy. Stir in 2 tablespoons lemon peel and 1/4 cup lemon juice. Stir in flour, cornstarch and salt. Press dough in pan with floured fingers.
3Bake 35 to 45 minutes or until top is light golden brown. Cool completely, about 45 minutes.
4Meanwhile, in medium bowl, stir all Lemon Icing ingredients until smooth and spreadable. Spread cooled bars with Lemon Icing; let stand 15 minutes or until set. For bars, cut into 6 rows by 6 rows.