Place the chicken breast in a small bowl with the buttermilk and marinate for at least 30 minutes in the refrigerator.
In a shallow dish, whisk together the whole wheat flour, paprika, garlic powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing excess to drip off, then dredge in the flour mixture, pressing firmly to create a thick, even coating.
Lightly brush or spray both sides of the coated chicken with avocado oil.
Place the chicken in an air fryer basket and cook at 375°F for 12-15 minutes, flipping halfway through, until the exterior is golden and the internal temperature reaches 165°F.
Toast the whole wheat bun until lightly browned, then assemble the sandwich with the crispy chicken, dill pickles, and romaine lettuce.