Add all 3 ingredients to a small blender like a Magic Bullet, or mini food processor. Blend until smooth, adding 1-2 teaspoons of water or non-dairy milk if needed to get things moving.
Taste for sweetness, and if preferred, add a teaspoon of maple syrup. (If your banana is ripe enough, you should not need added sweetener.)
Chill for 1 hour (or longer) for a colder treat, or enjoy it immediately.
Notes
The ripeness of your ingredients will determine if, and how much liquid you need to get things moving. Start blending without any added liquid and add as needed.
Make it a frozen chocolate treat by freezing it for 1 hour before enjoying it.