Place the chicken breast in a shallow bowl and cover with buttermilk to marinate for at least 20 minutes in the refrigerator.
In a separate small bowl, combine the whole wheat panko, garlic powder, smoked paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting any excess drip off, then dredge it thoroughly in the panko mixture until evenly coated.
Preheat your air fryer to 375 degrees Fahrenheit and spray the basket with avocado oil.
Place the chicken in the basket, spray the top of the chicken with avocado oil, and cook for 12 to 15 minutes, flipping halfway through, until the internal temperature reaches 165 degrees Fahrenheit.
While the chicken cooks, whisk together the Greek yogurt and Dijon mustard in a small bowl to create the sauce.
Toast the whole wheat bun until golden brown.
Assemble the sandwich by spreading the yogurt sauce on the bun, then layering the butter lettuce, tomato, crispy chicken, and dill pickles.