Mix sugar, sour cream, butter, and eggs in large mixer bowl on low for 1 minute.
Beat in mashed bananas and vanilla on low for 30 seconds.
Beat in flour, salt and soda on medium for 1 minute.
Stir in nuts. Spread in pan. Bake until light brown, 20-25 minutes. Cool.
For frosting, heat butter over medium heat until delicate brown color. Remove from heat. Mix in powdered sugar. Beat in vanilla and milk. Mix until smooth and spreadable.
Frost cooled bars.