Add all the ingredients to a blender. (If desired, add a handful of ice cubes for a colder smoothie.)
Blend all the ingredients together until smooth and creamy. If needed, scrape down the sides of the blender and blend again to ensure everything is well combined.
If the smoothie is too thick, add a little more milk. If it's too thin, add a few more ice cubes or another banana slice to thicken it up.
Pour the chocolate peanut butter banana smoothie into a glass or to go cup.