Empty flesh of avocados in large mixing bowls. You want to use very soft avocados.
Mash a bit with a fork and then add cocoa powder, about two tbsp per regular sized avocado or to taste. Also optionally add a sweetener; I've used powdered stevia, which has neglible effects on glucose levels. Then mash some more so that powders won't fly in next step.
Blend thoroughly using an immersion blender.
Transfer individual portions to ramekins and chill overnight.