Heat oven to 350°F. Grease 15x10x1-inch pan with shortening; lightly flour. In large bowl with electric mixer, beat bar ingredients on low speed until moistened. Beat 2 minutes on medium speed. Spread batter in pan.
Bake 20 to 30 minutes or until toothpick inserted in center comes out clean. Cool completely, about 1 hour.
Spread frosting over cooled bars. Refrigerate until set, about 30 minutes. Cut into 8 rows by 6 rows. Store in refrigerator.