Preheat the air fryer to 350°F.
Spread softened butter on the outside of both slices of bread — this is what gets you that golden, crispy exterior.
Flip the bread over and spread mustard on the inside of one or both slices, to taste.
Layer the ham and Swiss cheese on one slice of bread. Top with the second slice, butter-side up.
Place the sandwich in the air fryer basket butter-side down. Air fry at 350°F for 5 minutes, or until the bread is golden and the cheese is fully melted.
Remove carefully, slice in half, and serve immediately. Enjoy!