In a small saucepan, bring butter and water to a boil. Remove from the heat; set aside. In a large bowl, beat the sugars, bananas, buttermilk, eggs and vanilla until blended. Combine flour and baking soda; gradually add to sugar mixture. Beat in butter mixture until blended. Stir in nuts.
Pour into a greased 15x10x1-in. baking pan. Bake at 350° for 18-22 minutes or until a toothpick inserted in the center comes out clean. Cool on a wire rack.
For frosting, in a large bowl, beat cream cheese and butter until light and fluffy. Beat in vanilla. Gradually add confectioners' sugar until smooth. Spread over bars. Store in the refrigerator.