Place a Hobo Pie toaster in the hot coals of a campfire to preheat for a few minutes.
Butter both slices of bread, then place one slice, butter-side down into the toaster. Crack the egg onto the bread, and break the yolk. Top with a slice of cheese and the second slice of bread, butter-side up.
Close the toaster, and place into the coals of the fire. Cook for several minutes, turning frequently, until the bread has browned nicely on both sides.