Preheat air fryer to 370°F (188°C) for 3-5 minutes.
Spread butter evenly on one side of each slice of bread.
Place cheddar and mozzarella cheese between the unbuttered sides of the bread to make a sandwich.
Put the sandwich in the air fryer basket, buttered side out.
Air fry for 4-5 minutes, flip, and cook for another 3-4 minutes until golden brown and crispy.
Remove, slice, and serve hot.