Place the chicken breast between two pieces of parchment paper and pound with a meat mallet until it is an even half-inch thickness.
In a shallow bowl, combine the buttermilk with half of the garlic powder, onion powder, salt, and pepper, then submerge the chicken to marinate for at least 15 minutes.
In a separate shallow dish, whisk together the whole wheat flour, paprika, and the remaining spices until well combined.
Remove the chicken from the buttermilk, letting any excess drip off, then dredge thoroughly in the flour mixture, pressing to ensure a thick coating.
Lightly coat the air fryer basket with olive oil spray, place the chicken inside, and spray the top of the chicken generously before air frying at 375 degrees for 12 to 15 minutes, flipping halfway through.
Toast the whole wheat bun until lightly browned, then assemble the sandwich by layering the crispy chicken, fresh romaine lettuce, and tomato slices.