Heat oven to 375 degrees. Spray square pan, 9 X 9 X 2 inches, with nonstick cooking spray. Beat sugar, sour cream, margarine and egg whites in large bowl on low speed 1 minute, scraping bowl occasionally. Beat in bananas and vanilla on low speed 30 seconds. Beat in flour, salt and baking soda on medium speed 1 minute, scraping bowl occasionally. Stir in nuts. Spread in pan. Bake 20 to 25 minutes or until light brown; cool. Spread with Frosting. Cut into 2-1/4 X 1-1/2-inch bars. 24 BARS. FROSTING Mix all ingredients until smooth.