Pound the chicken breast to an even half-inch thickness using a meat mallet.
Place the chicken in a shallow bowl with the buttermilk and marinate for at least 20 minutes in the refrigerator.
In a separate bowl, whisk together the whole wheat flour, garlic powder, paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting excess drip off, and dredge thoroughly in the seasoned flour mixture.
Lightly coat both sides of the breaded chicken with avocado oil spray.
Place the chicken in the air fryer basket and cook at 375°F for 12 to 15 minutes, flipping halfway through, until the internal temperature reaches 165°F.
While the chicken cooks, stir together the nonfat Greek yogurt and Dijon mustard to create a tangy spread.
Toast the whole grain bun in a dry pan or toaster until lightly browned.
Assemble the sandwich by spreading the yogurt sauce on the bun and layering with lettuce, tomato, pickles, and the crispy chicken.