Preheat oven to 350° and spray a 15 x 10 jelly roll pan or a 13 x 9 baking dish with a non stick spray. Set aside.
In a medium mixing bowl, beat butter and sugar together until creamy.
Beat eggs, one at a time, into mixture.
Add sour cream and vanilla and mix well.
Combine flour, baking soda and salt in a small bowl and stir into wet ingredients.
Add mashed bananas, stir.
Pour batter into a 15 x 10 jelly roll pan or a 13 x 9 baking dish. *If using a jelly roll pan, bake 22-27 minutes, or 35-40 minutes in a baking dish, or until wooded toothpick come out clean. Remove from oven. Cool completely then frost.