
Preheat a cast iron skillet over medium heat. Butter the four slices of bread on one side each.
Place 2 slices of bread, butter side down, in the skillet. Spread the mayonnaise on top of the bread. Add two slices of ham and two slices of cheese to each slice of bread, on top of the mayo.
Spread mustard on the dry side of the other two slices of bread. Place the slices, mustard side down, on top of the cheese. Using a spatula to press down the sandwiches, cook the sandwiches until they are golden brown on each side. Serve immediately.