We had suspected it might be as from when he was tiny he vomited at the tiniest bit of milk or dark chocolate, but then he is also allergic to nuts and so there is always the issue with contamination - and by the time I found Kinnerton he associated it with vomiting so wouldn't try it.
Oddly enough though he can eat white chocolate without a reaction.
I have not come across anyone with a cocoa allergy before now and was wondering how common it is?