Preheat oven to 350 degrees F.
In a large mixer bowl, combine all bar ingredients except walnuts and carrots. Beat at low speed, scraping bowl often, until well mixed (1 to 2 minutes). By hand, stir in walnuts and carrots.
Spread into greased 15 x 10 x 1 inch jelly roll pan. Bake for 20 to 25 minutes or until top springs back when touched lightly in center. Cool completely
In a small mixer bowl, combine all frosting ingredients. Beat at medium speed, scraping bowl often, until smooth (1 to 2 minutes). Frost cooled bars; cut into bars. Store in refrigerator.