Place the chicken breast in a shallow bowl and pour the buttermilk over it, ensuring it is well coated.
In a separate small bowl, whisk together the whole wheat flour, garlic powder, onion powder, smoked paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting excess drip off, then dredge it thoroughly in the flour mixture until fully coated.
Preheat your air fryer to 375°F and lightly mist the basket with avocado oil.
Place the chicken in the basket, spray the top with the remaining avocado oil, and cook for 12-15 minutes, flipping halfway through, until golden and crispy.
Toast the whole wheat bun and assemble the sandwich by layering the crispy chicken, romaine lettuce, and tomato slices.