In a large mixing bowl, beat the butter with an electric mixer on medium to high speed for 30 seconds. Gradually add sugar, 2 tablespoons at a time. Beat about 6 minutes or until the mixture is very light and fluffy, scraping sides of bowl occasionally. Add vanilla. Add eggs, one at a time, beating for 1 minute after each addition. Add banana and chocolate, beating until combined. Add flour mixture and milk alternately to butter mixture, beating after each addition just until combined.