Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
Place the chicken in a shallow bowl with the buttermilk and marinate for at least 20 minutes in the refrigerator.
In a separate shallow dish, whisk together the whole wheat flour, panko breadcrumbs, garlic powder, smoked paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting any excess drip off, then dredge thoroughly in the flour and panko mixture, pressing to adhere.
Lightly mist the chicken on both sides with avocado oil and place in the air fryer basket.
Air fry at 400°F for 12 to 15 minutes, flipping halfway through, until the coating is golden brown and the internal temperature reaches 165°F.
While the chicken cooks, whisk the Greek yogurt and Dijon mustard together in a small bowl to create a clean-eating spread.
Toast the whole grain bun lightly, spread the yogurt sauce on both sides, and assemble the sandwich with the crispy chicken, butter lettuce, and tomato slices.