1Heat oven to 350°F.
2In medium bowl, mix flour, butter and 1/4 cup powdered sugar with spoon. Press in ungreased 8- or 9-inch square pan, building up 1/2-inch edges.
3Bake 20 minutes. Meanwhile, in medium bowl, beat remaining ingredients except additional powdered sugar with electric mixer on high speed about 3 minutes or until light and fluffy. Pour over hot crust.
4Bake 25 to 30 minutes or until no indentation remains when touched lightly in center. Cool completely in pan on cooling rack, about 1 hour. Sprinkle with powdered sugar. For bars, cut into 5 rows by 5 rows.