Preheat your air fryer to 370°F (188°C) for 3 minutes.
Spread 1 tablespoon of softened butter evenly on one side of each bread slice.
Flip one bread slice over and place the cheddar and American cheeses evenly on the unbuttered side, then top with the second bread slice with the buttered side facing outward.
Lightly spread the mayonnaise on the top outer surface of the sandwich.
Carefully place the sandwich in the air fryer basket, mayonnaise-side up. Air fry for 5-6 minutes.
Carefully flip the sandwich using tongs, and air fry for an additional 3-4 minutes until golden brown and crispy.
Remove the sandwich from the air fryer, let it rest for 1 minute on a cutting board, then slice diagonally and serve immediately.
Notes
Add mozzarella or feta for a cheesy twist. Let sandwich rest for 1 minute before slicing.