Blitz the avocado and banana in a smoothie maker or blender. Add cocoa powder to taste and blitz again. Add the yogurt and blitz again until super-smooth. Sweeten with honey if nescessary and add some of the chilled water until the smoothie is the right texture for you.
The type of cocoa powder you use will affect how much is needed, so add it gradually at first. If all the ingredients come straight from the fridge, you don't need to add crushed ice or chill.
*Inspired by Gordon E. featured in the Realfood Cookbook