It is a wonderful moist cake that seriously doesn't need any frosting. It rarely lasts long enough! But if I do decide to serve for company, I use a cream cheese frosting.
I often double the recipe, pour into a 9 by 13 inch pan, and make muffins out of extra batter.
Preheat the oven to 350 degrees F.
Butter and lightly flour a 9-inch square cake pan. Cream the butter, slowly add the sugar, and beat until light. Add the bananas, eggs, and vanilla, and beat well. Mix the flour, baking soda, and salt, add to the first mixture, and blend.
Slowly add the sour cream and beat until well blended. Spread in the pan and bake for about 45 minutes, or until a toothpick comes out clean. Cool in the pan for 5 minutes before turning out onto a rack.