In a large freezer bag, place chicken legs and buttermilk. Allow chicken to soak in buttermilk for at least 2 hours to overnight in the refrigerator.
Preheat air fryer to 390 degrees. Spray air fryer basket with nonstick cooking spray.
Mix together panko, flour, baking powder, and dry seasonings in a shallow dish.
Remove chicken from buttermilk and roll in panko mixture.
Evenly space chicken in air fryer basket. (Two or more batches will be required.)
Cook in the air fryer for 15 minutes. Remove from air fryer and brush with melted butter. Turn the chicken legs over. Return to air fryer.
Cook for an additional 10 to 15 minutes (depending on the thickness of the chicken legs.) Check chicken for doneness with an instant thermometer (165 degrees without touching the bone.)
Keep chicken warm until all chicken is cooked through.