i'll repeat what i said at youtube for you guys..
in the 60's i did some back packing around Europe and in the UK bought a couple cans of cocoa drink that were self heating.those had the drink in the outside can and for heat a big candle in the inner can.bad idea as i could of bought cocoa drink anywhere and the cans were heavy.the taste was not all that great and i got the impression they were some sort of WW Two gizmo rebranded for cavers.