Place the chicken breast in a small bowl and pour the buttermilk over it, ensuring it is fully submerged; marinate in the refrigerator for at least 30 minutes.
In a shallow dish, whisk together the whole wheat flour, garlic powder, onion powder, paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing the excess to drip off, and dredge it thoroughly through the flour mixture until evenly coated.
Lightly spray the breaded chicken with olive oil and place it in the air fryer basket at 375°F for 12 to 15 minutes, flipping halfway through, until the exterior is golden and crispy.
While the chicken cooks, lightly toast the whole grain bun in a dry pan or toaster until warm and slightly browned.
Assemble the sandwich by placing the crispy chicken on the bottom bun, then top with the romaine lettuce, tomato slices, and pickle chips before closing with the top bun.