Break the graham crackers into pieces and place a layer in the bottom of a cast iron skillet. Break up the chocolate bars and put a layer on top of the crackers. Now add another layer of crackers and chocolate. Evenly distribute the marshmallows over it all. Place the skillet in a corner of the campfire, rotating the pan 1/4 turn every 3 minutes, until the chocolate is melted and the marshmallow are golden brown, about 12 minutes. Be sure to protect your hands from the fire at all times.
Alternately, the S'mores can be made in a pie pan in a preheated 250 degree F oven until the chocolate and marshmallows melt.