Cream butter and sugar. Add eggs one at a time and beat well after each one. Stir in vanilla and beat in bananas and sour cream. Mix well. Add flour, soda, and salt. Mix well.
Pour into a greased 13-in. x 9-in. baking dish. Bake at 350° for 35-40 minutes or until a toothpick inserted near the center comes out clean. Cool completely.
For frosting, mix ingredients very well together and spread on a completely cooled cake.