Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
In a shallow bowl, whisk together the buttermilk, garlic powder, onion powder, paprika, sea salt, and black pepper.
Submerge the chicken in the buttermilk mixture and allow it to marinate for at least 30 minutes in the refrigerator.
Place the panko breadcrumbs in a separate shallow dish and dredge the marinated chicken, pressing firmly to ensure an even coating.
Lightly coat the breaded chicken with avocado oil spray and place in the air fryer basket.
Cook at 375°F for 12 to 15 minutes, flipping halfway through, until the exterior is golden brown and the internal temperature reaches 165°F.
Toast the whole grain bun until lightly browned, then spread the mayonnaise on the bottom half.
Assemble the sandwich by layering the romaine lettuce, the crispy chicken breast, and the dill pickles before topping with the other bun half.