Slice the chicken breast into a thin, even cutlet and place it in a bowl with the buttermilk to marinate for 20 minutes.
In a shallow dish, whisk together the whole wheat flour, smoked paprika, garlic powder, onion powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting the excess drip off, then dredge it in the flour mixture, pressing firmly to ensure a thick coating.
Lightly spray the coated chicken on both sides with avocado oil spray.
Place the chicken in an air fryer basket and cook at 375°F for 12 to 15 minutes, flipping halfway through, until the exterior is golden and the internal temperature reaches 165°F.
During the last 2 minutes of cooking, place the split bun in the air fryer to toast until lightly browned.
Assemble the sandwich by placing the crispy chicken on the bottom bun and topping with lettuce and tomato slices.