In large bowl, cream butter and sugar. Beat in eggs, bananas and vanilla. Combine flour, baking soda and salt in separate bowl. Add to creamed mixture and mix well. Pour into greased 15 x 10 inch baking pan.
Bake at 350 degrees for 25 minutes or until toothpick inserted in center of pan comes out clean. Cool completely.
To make frosting, cream butter and cream cheese in a mixing bowl. Add confectioners' sugar and vanilla gradually. Beat well. Spread evenly over bars. Cut into 1-inch squares and serve.