Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
Place the chicken in a shallow bowl and pour the buttermilk over it, ensuring it is fully submerged. Marinate for at least 30 minutes.
In a separate shallow dish, whisk together the whole wheat flour, panko breadcrumbs, smoked paprika, garlic powder, onion powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing excess to drip off, then dredge it in the flour mixture, pressing firmly to ensure an even coating.
Lightly spray the chicken on both sides with avocado 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 brown and the internal temperature reaches 165°F.
While the chicken cooks, mix the Greek yogurt and Dijon mustard in a small bowl to create a healthy sandwich spread.
Lightly toast the whole-grain bun.
Spread the yogurt sauce on the bottom bun, then layer with the romaine lettuce, crispy chicken, and dill pickles before topping with the other half of the bun.