In large bowl, beat butter with electric mixer on medium speed 30 seconds. Gradually add sugar, about 1/4 cup at a time, beating well after each addition. Beat 2 minutes longer. Add eggs, one at a time, beating well after each addition. Beat in bananas and vanilla. On low speed, alternately add flour mixture, about a third at a time, and milk, about half at a time, beating just until blended.