It is quite difficult to find this kind of food in Bangkok, they are selling by using three wheels motorcycle. They modified the motorcycle and cut the fuel tank to a half and put on the motorcycle. They put charcoal into the half-cutted fuel tank to grill those foods.
Three wheels motorbike with half cutted fuel tank on it and grill with charcoal
Charcoal grill yam balls and purple yam balls
The charcoal grill yam ball is selling 2 baht one and the grill corn is selling 12 baht for one.