Another lil' afterschool snack popular with 16 year olds, I've been getting into the Bo Bo act lately myself.
The pushcart fellow also peddles khoai mì nướng, [grilled shredded cassava cakes] but I was here for the Bo.
Here's a quick version of the Banh Bo Dua process. It's basically a cake made of two charcoal-fired shells, stuffed with sweetened, shredded coconut. Nothing fancy, just plain grubery satisfaction though.
Always on the move, but you can't go wrong hanging out outside of highschool to wait for him. Just don't look to suspicious. 5.000 VND for two cakes.