Place the chicken breast in a shallow dish and pour the buttermilk over it, ensuring it is fully coated. Let it marinate for at least 15 minutes.
In a small bowl, whisk together the whole wheat flour, cornstarch, smoked paprika, garlic powder, onion powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting any excess drip off, then dredge it thoroughly in the flour mixture until evenly coated.
Lightly spray or brush the chicken with avocado oil and place it in the air fryer basket.
Air fry at 375°F (190°C) for 12-15 minutes, flipping halfway through, until the exterior is golden brown and the internal temperature reaches 165°F.
While the chicken cooks, whisk together the Greek yogurt and Dijon mustard in a small bowl to create the sauce.
Toast the whole grain bun lightly in a pan or toaster.
Spread the yogurt sauce on the bottom bun, then layer with the butter lettuce, tomato slice, crispy chicken, and dill pickles before topping with the other half of the bun.