Preheat oven to 350 degrees F Grease a 12x15 inch jellyroll pan.
In a large bowl, cream the butter and sugar together until smooth. Beat in the eggs, one at a time, along with the sour cream and vanilla. Combine the flour, baking soda and salt; and beat into the batter. Then mix in the mashed banana. Spread uniformly 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 , then apply the cream cheese frosting.
