Cocoa is a powder made from the seed from the cocoa tree. Since seeds fall into the category of food, it would not be allowed outside of the morning hours.
Chocolate also has narcotic properties, so could potentially be considered a medicine. Wikipedia says it may reduce cholesterol and ease diarrhea symptoms, so for people suffering from either of those ailments, it could be used at any time and kept for life, provided that was the reason for use.
It is quite surprising to hear that otherwise strict monks are using it as a refreshment in the evening; I think they consider the natural fatigue and perhaps even hunger in the evening to constitute "sickness" that chocolate "cures". It is quite clear, however, that the Buddha allowed only fruit and vegetable juice for that purpose and using the oil found in much chocolate for this purpose is an abuse. Sugar might be okay, since it is technically dehydrated juice, but that is still a bit of a stretch.