Preheat oven to 350 Grease a 10 x 15 jelly roll pan. Set aside.
In a large bowl, cream together the butter and sugar until smooth. Beat in the eggs, one at a time and mix thoroughly. Stir in sour cream, buttermilk and vanilla.
Combine the flour, baking soda and salt in a small bowl; stir into the batter. Mix in the mashed banana. Spread evenly into the prepared pan.
Bake for 20 to 25 minutes or until a toothpick inserted into the center comes out clean. Allow bars to cool completely before frosting with the cream cheese frosting.
To make the Frosting:
Mix butter and cream cheese in medium bowl until smooth and creamy. Add vanilla and powdered sugar a cup at a time and mix until smooth. Thin with milk if necessary.
Notes
We like these best cold. Store in an airtight container in the refrigerator.