Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
In a shallow bowl, soak the chicken in buttermilk for at least 20 minutes to tenderize.
In a separate bowl, whisk together the panko, flour, smoked paprika, garlic powder, onion powder, salt, and pepper.
Remove the chicken from the buttermilk, letting excess drip off, then dredge thoroughly in the seasoned breadcrumb mixture.
Preheat the air fryer to 400°F and coat the chicken generously with avocado oil spray.
Air fry the chicken for 12-15 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.
Lightly toast the whole wheat bun and spread the yogurt-mustard sauce on both sides.
Assemble the sandwich by placing the hot chicken on the bottom bun and topping with the crunchy pickle slices.