Freeze the banana for 2-3 hours before using. (see note) You can freeze some bananas days ahead of time so they're ready whenever you want a smoothie.
Add the banana, milk, ice, oats, peanut butter, and cocoa powder to a blender. Cover and puree until smooth.
Pour into a glass and serve immediately. If desired garnish with some banana slices, oats, cocoa powder, melted peanut butter, and/or chocolate chips.
If you prefer, you can use 2% or whole milk in place of almond milk...or another non-dairy milk you prefer.
Freeze the bananas ahead of time in slices around 2 inches thick. If you freeze the entire banana before cutting it, it will be too difficult to blend or chop. This is also a great way of using those very ripe bananas on your counter!