In a mixing bowl, cream butter and sugar. Add eggs, sour cream, and vanilla extract. Combine flour, baking soda and salt; gradually add to the creamed mixture. Stire in bananas. Spread into a greased jelly roll pan - 15 x 10 x 1. Bake at 350 degrees for 20-25 minutes or until a tooth pick inserted near the center comes out clean. Cool on a rack.
For Frosting: in a mixing bowl, beat cream cheese, butter and vanilla extract. Gradually beat in enough powdered sugar to achieve desired consistency. When bars are cool frost. Store in refridgerator.