Lay 2 of the waffles on a counter or cutting board. Place a half-slice of cheese on top of each waffle. Spread some mustard over the cheese if using. Fold each ham slice in half, and place on top of the cheese slices. Layer another half-slice of cheese over each piece of ham. Top with the remaining waffles to form a sandwich.
Heat a nonstick skillet or cast-iron pan over medium heat.
Grease the skillet with some butter. Place the sandwiches in the pan, and cook until golden on the underside and the cheese starts to melt, 2 to 3 minutes. Flip the sandwiches, and cook until golden on other side and the cheese fully melts, about 2 more minutes. Serve hot or wrap in foil to pack for a school lunch or picnic.
Make it Your Own: For an Italian spin on this sandwich, swap in prosciutto for the ham and mozzarella for the Swiss.
This recipe works great with store-bought frozen waffles, too.