An alternative to using butter in the recipe is to drizzle 1 tablespoon of olive oil over the top of each salmon filet and rub it over the surface.
Make sure your fillets have the same thickness so they cook the same.
It’s better to undercook than overcook, especially with a delicate food like salmon. If you find that your fish is not fully cooked in the center, pop it back into the air fryer for a couple minutes.