Add the banana, milk, cacao powder, peanut butter, and vanilla to a high-speed blender and blend until smooth and creamy.
Taste and adjust the texture as needed, adding a little more liquid if you need help blending, or a handful of ice if you'd like a thicker texture. Just keep in mind that adding extra ice or liquid will dilute the flavor slightly. Feel free to add a little more cacao powder for a stronger chocolate flavor, or a squeeze of honey or maple syrup, if you want a sweeter taste.
Serve right away for the best taste and texture. See the tips in the post above for making smoothie bags that you can store in the freezer, for a make-ahead idea.
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Notes
Nutrition information is for the whole batch, and is automatically calculated. This information is just an estimate, not a guarantee.