In a bowl, combine flour, bakng soda, salt and chocolate chips. In a larger bowl, cream together butter and sugar. Add eggs one at a time. Add vanilla and mashed banana. Add dry ingredients to moist mixture and combine, stirring until mixture is combined but still lumpy. Pour into a 12-cup muffin pan that has been greased only on the bottom and halfway up the sides of each cup. Bake at 350°F for 25 min.