Once the grill has preheated, place the cast iron pan directly on the grill grates and allow it to heat for 5-10 min while you get the sandwich ready.
Spread one side of each piece of bread with about 1/2 tablespoon of butter.
Place one piece of bread, buttered side down on a baking sheet or plate and lay the cheese on the bread. Close the sandwich with the other piece of bread (butter side out) and bring it and the remaining tablespoon of butter to the grill.
Place the butter in the hot skillet, and then lay the grilled cheese in the pan. Cook for about 8-10 min per side.