Place the chicken breast in a shallow bowl and cover with buttermilk; let marinate for 15 minutes.
In a separate small bowl, 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 until fully coated.
Preheat your air fryer to 375°F and lightly coat the basket with avocado oil spray.
Place the chicken in the basket, spray the top of the chicken lightly with avocado oil, and cook for 12-15 minutes, flipping halfway through, until the internal temperature reaches 165°F.
While the chicken cooks, whisk together the Greek yogurt and Dijon mustard to create a healthy sandwich spread.
Toast the whole wheat bun until lightly browned.
Assemble the sandwich by spreading the yogurt sauce on the bun, then layering the lettuce, tomato, pickles, and the crispy chicken breast.