Add about 1/3 cup milk to a small pot or pan and set it on top of the coals away from any flames. Whisk the milk until it starts to steam, about 5 minutes. Remove the pot from the campfire and stir in small broken-up pieces of milk chocolate from about three chocolate bars. Keep stirring until you've got a smooth chocolate liquid. Toast some marshmallows, then dip them into the melted chocolate and eat as is, or transfer them to graham crackers.