Place the chicken breast between two sheets of parchment paper and pound to an even thickness to ensure uniform cooking.
Submerge the chicken in the buttermilk in a shallow bowl and marinate for at least 15 minutes in the refrigerator.
In a separate shallow dish, whisk together the panko breadcrumbs, garlic powder, onion powder, paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing excess to drip off, then dredge thoroughly in the seasoned panko mixture until fully coated.
Lightly coat the chicken on both sides with the avocado oil using a spray or brush.
Place the chicken in the air fryer basket and cook at 400°F for 12-15 minutes, flipping halfway through, until the exterior is golden and the internal temperature reaches 165°F.
Lightly toast the whole grain bun in a pan or toaster until warm and fragrant.
Assemble the sandwich by placing the crispy chicken on the bottom bun and topping with romaine lettuce, tomato slices, and dill pickles.