In a large bowl, cream butter and sugar until light and fluffy. Add the eggs, sour cream and vanilla. Combine the flour, baking soda and salt; gradually add to the creamed mixture. Stir in bananas.
Spread into a greased 15-in. x 10-in. x 1-in. baking pan. Bake at 350° for 20-25 minutes or until a toothpick inserted near the center comes out clean (do not overbake). Cool.
For frosting, in a large bowl, beat the cream cheese, butter and vanilla until fluffy. Gradually beat in enough confectioners' sugar to achieve desired consistency. Frost bars. Store in the refrigerator.