Place the chicken breast in a shallow bowl and pour the buttermilk over it, ensuring it is fully submerged to marinate for at least 15 minutes.
In a separate shallow dish, combine the panko breadcrumbs, garlic powder, paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting any excess drip off, then dredge it thoroughly in the panko mixture, pressing firmly to adhere.
Lightly coat the breaded chicken on both sides with the avocado oil spray.
Place the chicken in the 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.
Toast the whole grain bun in the air fryer or a toaster until lightly browned.
Assemble the sandwich by placing the crispy chicken on the bottom bun, followed by the romaine lettuce, tomato slices, and pickle chips before topping with the other half of the bun.