Place the chicken breast in a shallow bowl and pour the buttermilk over it, ensuring it is fully submerged. Let it marinate for at least 15 minutes.
In a separate shallow dish, combine the 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 through the panko mixture, pressing firmly to adhere the crumbs to all sides.
Preheat your air fryer to 375°F. Lightly spray the air fryer basket with avocado oil.
Place the breaded chicken in the basket and spray the top of the chicken with avocado oil. Air fry for 12-15 minutes, flipping halfway through, until the internal temperature reaches 165°F and the crust is golden brown.
While the chicken cooks, whisk together the Greek yogurt and Dijon mustard in a small bowl to create a high-protein spread.
Toast the whole wheat bun lightly in the air fryer or a toaster.
Assemble the sandwich by spreading the yogurt-mustard sauce on the bun, then layering the lettuce, tomato, crispy chicken, and pickles.