Preheat your air fryer to 350 to 375°F. Lay 3 to 4 bacon slices in a single layer in the basket (no overlap if you can help it).
Air fry the bacon for 3 to 6 minutes at 350 to 375°F, until it’s as crisp as you like. Transfer to a plate and blot lightly if there’s excess grease.
While the bacon cooks, fry 2 eggs in a skillet, seasoning lightly if you want. Keep the eggs roughly the size of your bread (fold if needed). Flip once so both sides are set.
Spread 1 tablespoon butter (or mayo) on one side of each bread slice. If using mayo as a condiment inside the sandwich, spread up to 1 teaspoon on the inside of one slice.
Assemble: place 1 slice of bread down, add 1 slice cheddar, add the eggs, then another slice cheddar. Add the bacon, then add remaining cheddar (if using), and top with the second bread slice.
Place the assembled sandwich back in the air fryer and cook for 1 to 2 minutes at 350 to 375°F, just until the cheese is melted and the sandwich is hot and lightly crisped.
Remove, slice if you want, and eat right away while the cheese is gooey and the bacon stays crisp.