Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
In a shallow bowl, soak the chicken in the buttermilk for at least 10 minutes.
In a separate bowl, whisk together the whole wheat flour, cornstarch, garlic powder, onion powder, paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting excess drip off, then dredge thoroughly in the flour mixture, pressing to adhere.
Lightly spray the chicken on both sides with avocado oil and place in the air fryer basket.
Air fry at 375°F for 12-15 minutes, flipping halfway through, until the internal temperature reaches 165°F and the exterior is crispy.
While the chicken cooks, mix the Greek yogurt and Dijon mustard in a small bowl to create the sauce.
Toast the whole wheat bun until lightly browned.
Assemble the sandwich by spreading the sauce on the bun, then layering the lettuce, tomato, and the crispy chicken.