Place the chicken breast between two sheets of parchment paper and pound to an even 1/2-inch thickness for uniform cooking.
Submerge the chicken in the buttermilk in a shallow bowl and let it marinate for at least 15 minutes to ensure a tender interior.
In a separate shallow dish, whisk together the whole wheat flour, smoked paprika, garlic powder, onion powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing excess to drip off, and dredge thoroughly in the flour mixture, pressing firmly to create a thick coating.
Lightly mist the coated chicken with the olive oil and place it in the air fryer basket.
Air fry at 375°F for 12-15 minutes, flipping halfway through, until the exterior is golden and crunchy and the internal temperature reaches 165°F.
While the chicken cooks, stir the Greek yogurt and Dijon mustard together in a small bowl to create a creamy, zesty spread.
Toast the whole wheat bun until warm, then assemble the sandwich by spreading the sauce on the bun and layering with lettuce, tomato, and the crispy chicken.