In a mixing bowl, cream butter and sugar until light and fluffy.
Beat in eggs, sour cream, and vanilla until well blended.
Stir in mashed bananas.
Combine flour, baking soda, and salt; gradually add to the banana mixture.
Spread into a greased 15x10x1-inch baking pan.
Bake for 20-25 minutes or until a toothpick inserted in the center comes out clean. Cool in pan on a wire rack.
For frosting, in a mixing bowl, beat butter until creamy. Gradually add powdered sugar, vanilla, and enough milk to achieve spreading consistency. Frost bars.