Heat oven to 350°F. In medium bowl, mix flour, butter and 1/4 cup powdered sugar with spoon until well mixed. Press in ungreased 8- or 9-inch square pan, building up 1/2-inch edges.
Bake crust 20 minutes; remove from oven. 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.
Bake 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. Cut into 5 rows by 5 rows.