Preheat oven to 350 degrees F (175 degrees C). Grease and flour a 9x9 inch pan.
In a large bowl, cream butter and sugar until light. Add eggs and beat well. Dissolve soda in the sour cream and add it to the butter mixture. Beat well. Add the mashed bananas and mix in. Add cake flour and mix well. Stir in vanilla extract.
Pour batter into a 9x9 inch pan and bake at 350 degrees F (175 degrees C) for 45 minutes or until done.