That's a good question. I assumed that cocoa existed in Ancient India, but it's native to Mexico, so that doesn't really make sense. The best explanation I could find just from Googling it seems to be that this is actually an interpretation specific to Thailand of a more general rule:
FAQ 9: "In Thailand, it has been observed that Thai Buddhist monks are allowed to drink tea, cocoa, coffee (but without milk) after midday. But in some other Buddhist countries like Burma, monks are not allowed to do this. Is this part of the Vinaya rules or is this just tradition, custom, or local practice? If it is in the Vinaya, how do you explain the differences in interpretation?"
A: The fourth of the Recollections of the Bhikkhu's Requisites is:
"Properly considering medicinal requisites for curing the sick, I use them: simply to ward off any pains of illness that have arisen, and for the maximum freedom from disease." (OP p.47)
There is an allowance in the Paali texts that 'medicinal-tonics' can be taken in the afternoon while 'lifetime-medicines' may be consumed any time they are needed. (See Lifetime Medicines.)
There are different interpretations and practices about how ill a bhikkhu has to be for it to be allowable to take such 'medicines.' Some bhikkhus will not take anything other than pure water, while some will over-stretch the Rule to even drinking 'medicinal' food-drinks (e.g., Ovaltine) in the afternoon. Some bhikkhus will consider tea-leaves allowable (as 'herbs') while some will see it as food or as a 'stimulant' (caffeine) and therefore not appropriate. Also, the ordinary rural villagers of South East Asia (until very recently) would have had no tea or coffee to drink, so such items could be considered quite a luxury. It will depend on local conditions and interpretations, which are allowed for in the Vinaya through the Great Standards. (See also Lifetime Medicines.)
FAQ 9: "In Thailand, it has been observed that Thai Buddhist monks are allowed to drink tea, cocoa, coffee (but without milk) after midday. But in some other Buddhist countries like Burma, monks are not allowed to do this. Is this part of the Vinaya rules or is this just tradition, custom, or local practice? If it is in the Vinaya, how do you explain the differences in interpretation?"
A: The fourth of the Recollections of the Bhikkhu's Requisites is: "Properly considering medicinal requisites for curing the sick, I use them: simply to ward off any pains of illness that have arisen, and for the maximum freedom from disease." (OP p.47)
There is an allowance in the Paali texts that 'medicinal-tonics' can be taken in the afternoon while 'lifetime-medicines' may be consumed any time they are needed. (See Lifetime Medicines.)
There are different interpretations and practices about how ill a bhikkhu has to be for it to be allowable to take such 'medicines.' Some bhikkhus will not take anything other than pure water, while some will over-stretch the Rule to even drinking 'medicinal' food-drinks (e.g., Ovaltine) in the afternoon. Some bhikkhus will consider tea-leaves allowable (as 'herbs') while some will see it as food or as a 'stimulant' (caffeine) and therefore not appropriate. Also, the ordinary rural villagers of South East Asia (until very recently) would have had no tea or coffee to drink, so such items could be considered quite a luxury. It will depend on local conditions and interpretations, which are allowed for in the Vinaya through the Great Standards. (See also Lifetime Medicines.)
https://www.accesstoinsight.org/lib/authors/ariyesako/laygui...