Preheat a skillet or griddle over medium heat.
Slice each pretzel bun in half horizontally.
Spread 1 tablespoon of Dijon mustard evenly on the bottom half of each bun.
Layer 2 slices of sliced deli ham and 2 slices of Swiss cheese on each bottom bun half.
Place the top bun halves over the cheese layers.
Spread butter on the outsides of the sandwich buns.
Place the sandwiches in the skillet and toast for about 4-5 minutes on each side, pressing gently, until the buns are golden brown and the cheese has melted.
Remove from heat and let rest for a minute before serving.
Cut each sandwich in half and stack the halves slightly offset on a wooden board.
Serve with a small ramekin of Dijon mustard on the side.