Place the butter and sugar into a large bowl. Beat with a mixer until the ingredients are creamy, light and fluffy. Add the eggs, sour cream and vanilla to the bowl, beat with a mixer until well incorporated. Combine the flour, baking soda and salt in a medium bowl, stir to combine. Gradually add the flour mixture to the batter, beating until the flour mixture is just incorporated. Stir in the mashed bananas. Spread the batter into a greased jelly roll pan. Bake at 350 degrees for 20 - 25 minutes or until a toothpick inserted in the center comes out clean. Do not over bake the bars. Cool the bars completely.
Cream Cheese FrostingUsing a large bowl, beat the cream cheese, butter and vanilla until fluffy. Gradually beat in enough confectioners sugar to achieve your desired spreading consistency. Frost the bars.