Place the chicken breast between two sheets of plastic wrap and pound to an even half-inch thickness.
Submerge the chicken in buttermilk in a shallow bowl and let marinate for at least 20 minutes in the refrigerator.
In a separate shallow dish, combine the panko breadcrumbs, smoked paprika, garlic powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing excess to drip off, then dredge in the panko mixture, pressing firmly to ensure an even coating.
Lightly brush or spray the breaded chicken with olive oil and place in the air fryer basket.
Air-fry at 400°F for 12-15 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.
Assemble the sandwich by placing the crispy chicken on the bottom bun, followed by the lettuce, tomato slices, and pickles before adding the top bun.