Place 4 of the waffles on a work surface. Spread one side of each with the mustard (if using).
Top with the ham, cheese, and remaining waffles. Spread the top of each sandwich with ½ tablespoon of the butter.
Melt the remaining 2 tablespoons butter in a large nonstick skillet over medium heat. Add the sandwiches, buttered-side up.
Cook, pressing occasionally with the back of a spatula, until the cheese melts and the waffles are golden, 3 to 4 minutes on each side.