This recipe comes from the first cookbook I ever had -- a children's cookbook whose name has long been lost. This is still my favorite recipe for banana smoothies -- it's incredibly easy, tasty, and decently healthy. Don't let the nutrition facts fool you -- this recipe is nice and sweet. The riper your banana, the sweeter it will be. If you REALLY have a sweet tooth, you could add sugar, but I never bother -- the vanilla and banana make it sugary enough.