Place the chicken breast between two sheets of parchment paper and pound to an even 1/2-inch thickness.
In a shallow bowl, whisk together the buttermilk, garlic powder, paprika, sea salt, and black pepper.
Submerge the chicken in the buttermilk mixture and let it marinate for at least 15 minutes in the refrigerator.
Place the panko breadcrumbs in a separate shallow dish and press the marinated chicken into the crumbs until fully and evenly coated.
Lightly coat the chicken on both sides with olive oil spray and place in the air fryer basket.
Air fry at 375°F for 12 to 15 minutes, flipping halfway through, until the exterior is golden and the chicken is cooked through.
In a small bowl, stir together the Greek yogurt and Dijon mustard until smooth.
Toast the whole grain bun until lightly browned, then spread the yogurt sauce on the bottom half.
Assemble the sandwich by layering the lettuce, crispy chicken, and dill pickles onto the bun.