To prepare the sandwich, stack 4 slices of ham and 2 slices of cheese on a slice of bread. Evenly apply a quarter portion of the mustard to the inside of the second slice of bread. After stacking evenly, apply butter to both exterior slices, and then cook in a preheated pan or griddle on medium heat. Cook until golden brown, and then flip and repeat the process. This should take 2 to 3 minutes per side. Once cooked, remove and enjoy.