Place the chicken breast between two sheets of parchment paper and pound with a meat mallet until it is an even half-inch thickness.
Place the chicken in a shallow bowl and pour the buttermilk over it, ensuring it is fully submerged. Let it marinate for at least 20 minutes in the refrigerator.
In a separate shallow dish, combine the panko breadcrumbs, garlic powder, onion powder, paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting any excess drip off, and dredge it thoroughly in the seasoned panko mixture, pressing firmly to adhere.
Lightly spray the air fryer basket with olive oil. Place the chicken in the basket and spray the top of the chicken with the remaining olive oil.
Air fry at 375°F (190°C) for 12-15 minutes, flipping halfway through, until the coating is golden brown and the internal temperature reaches 165°F.
During the last 2 minutes of cooking, place the split whole wheat bun in the air fryer to lightly toast.
Assemble the sandwich by placing the crispy chicken on the bottom bun, topping with romaine lettuce and tomato slices, and finishing with the top bun.