Add it to the blender along with the banana and cacao.
Blend until smooth then add a little water and blend again until you've reached the desired consistency.
Sweeten to taste with honey or maple syrup
Notes
You may find that you need more or less cacao depending on the brand that you're using. I have no idea why there seems to be such a difference in the flavour intensity between brands.
Recipe by A Life of Geekery at http://www.alifeofgeekery.co.uk/2015/04/healthy-chocolate-shake.html