Place the chicken breast between two sheets of parchment paper and pound with a meat mallet until it is an even half-inch thickness.
Place the chicken in a shallow bowl and pour the buttermilk over it, ensuring it is fully submerged, and let it marinate for at least 15 minutes.
In a separate shallow dish, whisk together the whole wheat flour, panko breadcrumbs, smoked paprika, garlic powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting any excess drip off, then dredge it thoroughly in the breadcrumb mixture, pressing down firmly to adhere the coating.
Lightly brush or spray the breaded chicken with avocado oil and place it in an air fryer basket.
Air fry at 375°F for 12 to 15 minutes, flipping halfway through, until the internal temperature reaches 165°F and the exterior is golden brown.
While the chicken cooks, whisk the Greek yogurt and Dijon mustard together in a small bowl to create a light sandwich spread.
Toast the whole wheat bun until lightly browned, then assemble the sandwich by spreading the yogurt sauce on the bun and layering with lettuce, the crispy chicken, and pickle slices.