Heat oven to 350°F. Grease 15x10x1-inch baking pan. In large bowl, combine all bar ingredients at low speed until moistened; beat 2 minutes at medium speed. Pour into greased pan.
Bake at 350°F. for 25 to 30 minutes or until toothpick inserted in center comes out clean. Cool 1 hour or until completely cooled.
In small bowl, combine all frosting ingredients; beat until smooth. Spread over cooled bars. Cut into bars. Store in refrigerator.