Heat oven to 350. Line a 13x9 pan with foil so that about 1" of foil goes over the edge of the pan. Grease foil generously.
Combine butter, sugar, peanut butter, egg and vanilla.Add flour and baking soda. Add in oats, banana, chips and peanuts.
Place mixture into prepared pan. Bake for about 25 minutes or until golden brown. Place on rack and cool completely.
To make frosting, place butter in bowl and mix until creamy. Add powdered sugar. Combine well. Add milk, combine. Add cocoa and continue beating until frosting is light and fluffy. You may add additional milk to get consistency that you like. Frost cooled bars. Remove from pan by lifting using the overhanging foil. Cut into squares.