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 in the refrigerator for at least 30 minutes.
In a separate shallow dish, whisk together the flour, cornstarch, paprika, garlic powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting the excess drip off, and dredge thoroughly in the flour mixture, pressing to adhere.
Lightly brush or spray both sides of the coated chicken with avocado oil.
Place in the air fryer 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 bowl, stir together the Greek yogurt and Dijon mustard to create a creamy sauce.
Toast the sprouted grain bun until warm, spread the yogurt sauce on the bottom, and assemble with lettuce, tomato, and the crispy chicken.