Heat oven to 350 degrees. Combine dry ingredients in a separate bowl (flour, soda, salt) and set aside. Cream butter and sugar in a large bowl. Add eggs, sour cream and vanilla. Mix well. Add dry ingredients to moist ingredients. Add mashed banana. Pour into a greased jelly roll pan (15 X 10 X 1"). Bake at 350 degrees for about 20-25 minutes (do not overbake). Cool completely (it helps to put the pan on a wire rack to cool evenly).
Frosting: Combine cream cheese, butter and vanilla. Mix well. Add enough confectioner's sugar to make the frosting as spreadable as you desire. Spread over cooled bars.
Store tightly covered and refrigerate unused bars within several hours.