Add the sliced banana, peanut butter, brown sugar, vanilla extract, and milk to a blender.
Blend the ingredients until smooth. If the smoothie is too thick, add more milk. If the smoothie is too thin, add more frozen banana. Taste and adjust the sweetness to your liking.
This smoothie recipe is incredibly flexible. You can use any type of dairy or non-dairy milk, keeping in mind that the creamier the milk, the creamier the smoothie will be. I like whole milk the best, but I think coconut would also pair nicely with these flavors!
I love the classic peanut butter-banana combo, but if you don't like peanut butter or can't have it, feel free to substitute it with any type of nut or seed butter. Almond butter or Sun Butter would be awesome, and it would also be fun to experiment with flavored nut butters. The nut butter makes this smoothie extra rich and delicious, so don't skip this ingredient!