Slice the chicken breast into a flat fillet and marinate in buttermilk for at least 30 minutes in the refrigerator.
In a shallow bowl, combine the panko breadcrumbs, sea salt, black pepper, smoked paprika, and garlic powder.
Remove the chicken from the buttermilk, letting excess drip off, and dredge in the panko mixture, pressing firmly to ensure a thick, even coating.
Lightly coat the breaded chicken with avocado oil using a spray or brush.
Place the chicken in the air fryer and cook at 375°F for 12-15 minutes, flipping halfway through, until the exterior is golden and crisp.
In a small bowl, whisk together the plain Greek yogurt and Dijon mustard to create the sandwich spread.
Toast the whole wheat bun until lightly browned and warm.
Assemble the sandwich by spreading the yogurt sauce on both sides of the bun and layering with the butter lettuce, tomato slice, pickle chips, and the crispy chicken fillet.