Place the chicken breast between two sheets of parchment paper and pound with a meat mallet until it reaches a uniform thickness of about half an inch.
In a shallow bowl, submerge the chicken in the low-fat buttermilk and let it marinate in the refrigerator for at least 30 minutes to tenderize.
In a separate wide dish, whisk together the whole wheat flour, cornstarch, garlic powder, onion powder, smoked paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting the excess liquid drip off, then dredge thoroughly in the flour mixture, pressing firmly to ensure a thick, even coating.
Lightly brush or spray the coated chicken with avocado oil and place it in the air fryer basket at 375°F for 12 to 15 minutes, flipping halfway through until golden brown.
While the chicken is cooking, lightly toast the whole grain bun and spread the nonfat Greek yogurt onto the bottom half.
Assemble the sandwich by layering the romaine lettuce, the crispy chicken breast, and the dill pickles onto the prepared bun.