Stir together butter and mustard. Spread every other slice of bread with slightly less than 2 teaspoons mustard mixture, creating 6 sandwiches. Fold each slice of cheese in half diagonally; tuck each into sandwich. Divide ham evenly among sandwiches, tucking in to fit. Place loaf on center of foil. Seal edges, making tight 1/2-inch fold; fold again, allowing space for heat circulation and expansion.