Place the chicken breast in a shallow bowl and pour the buttermilk over it, ensuring it is fully submerged; let it marinate for at least 30 minutes in the refrigerator.
In a separate wide dish, whisk together the whole wheat flour, smoked paprika, garlic powder, onion powder, sea salt, and black pepper until well combined.
Remove the chicken from the buttermilk, allowing the excess to drip off, then dredge it thoroughly in the flour mixture until completely and evenly coated.
Preheat your air fryer to 375°F and lightly coat the basket with half of the avocado oil spray.
Place the coated chicken in the air fryer, spray the top of the chicken with the remaining avocado oil spray, and cook for 12-15 minutes, flipping halfway through, until the exterior is golden and crispy.
Lightly toast the whole wheat bun in a pan or toaster, then assemble the sandwich by layering the romaine lettuce, tomato slices, and the crispy chicken on the bottom bun before closing.