Spread 1/2 tablespoon of mustard onto each of 4 slices of marble rye bread.
Add 2 ounces (1/2 cup) of smoked Gouda, followed by 2 ounces of black forest ham.
Cover each prepared bread slice and evenly butter the top of each sandwich. Place each sandwich butter side down in a frying pan over medium heat. While the first side is grilling butter second side of bread. Cook until golden and crusty, about 3-4 minutes. Carefully turn and cook until second side is golden, about 2-3 minutes more.