Remove the pips from the avocados and scoop the pulp into the food processor/blender.
Add the cocoa, maple syrup, vanilla and milk.
Blend and blend until all the lumps and bumps are gone... then blend some more.
Taste the mousse at this stage. If the taste of avocado is too strong add 1 Tablespoon extra each of cocoa and maple syrup. Blend and retaste. Repeat again if necessary (remembering the taste of the mousse will mellow and improve upon resting)
The mousse is great to eat now, but for a lighter consistency transfer to a stand mixer with a whisk attachment.
Add the extra milk and whisk for 3 minutes, or until fluffy.
The mousse will keep in the fridge for 3 days... but will most likely be eaten much quicker!