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 the remaining waffles. Spread the top of each sandwich with 1/2 tablespoon of the butter. Melt the remaining butter in a large nonstick skillet over medium heat. Place the sandwiches in the skillet, 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.