Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness using a meat mallet.
Place the chicken in a shallow bowl and pour the buttermilk over it, ensuring it is fully submerged; marinate in the refrigerator for at least 30 minutes.
In a separate shallow dish, whisk together 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 it through the panko mixture, pressing firmly to create a thick crust.
Lightly spray the breaded chicken on both sides with avocado oil and place it in the air fryer basket.
Air-fry at 375°F for 12 to 15 minutes, flipping halfway through, until the internal temperature reaches 165°F and the exterior is golden brown.
Toast the whole wheat bun lightly, then assemble the sandwich by layering the lettuce, tomato, pickles, and the crispy chicken breast.