Preheat oven to 350 degrees F (175 degrees C). Grease a 10x15 inch jellyroll pan.
In a large bowl, cream together the butter and sugar until smooth. Beat in the eggs, one at a time, then stir in the sour cream and vanilla. Combine the flour, baking soda and salt; stir into the batter. Finally, mix in the mashed banana. Spread evenly into the prepared pan.
Bake for 20 to 25 minutes in the preheated oven, until a toothpick inserted into the center comes out clean. Allow bars to cool completely before frosting with the cream cheese frosting.