Place the chicken breast in a small bowl with the buttermilk and let marinate for at least 30 minutes in the refrigerator.
In a shallow dish, whisk together the whole wheat flour, garlic powder, paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing excess to drip off, then dredge thoroughly in the seasoned flour mixture until fully coated.
Lightly spray or brush the chicken with avocado oil and place in the air fryer basket.
Air fry at 375°F for 12-15 minutes, flipping halfway through, until the exterior is golden-brown and the internal temperature reaches 165°F.
In a small ramekin, stir together the Greek yogurt and Dijon mustard to create a creamy spread.
Toast the whole grain bun until warm and lightly crisp.
Spread the yogurt mixture on the bun, add the crispy chicken and pickles, and serve immediately.