Place the chicken breast between two pieces of parchment paper and pound with a meat mallet until it reaches an even half-inch thickness.
Submerge the chicken in a bowl with the buttermilk and let it marinate in the refrigerator for at least 30 minutes to tenderize.
In a shallow 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 any excess drip off, then dredge it thoroughly in the flour mixture, pressing firmly to adhere.
Lightly coat the breaded chicken on both sides with the avocado oil using a brush or oil sprayer.
Place the chicken in the air fryer basket and cook at 375°F for 12 to 15 minutes, flipping halfway through, until the exterior is golden and crispy.
While the chicken cooks, whisk the Greek yogurt and Dijon mustard together in a small bowl to create a zesty spread.
Toast the whole wheat bun until lightly browned, then assemble the sandwich by layering the lettuce, crispy chicken, pickles, and yogurt sauce.