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 15 minutes to tenderize.
In a separate shallow dish, combine panko, garlic powder, onion powder, paprika, salt, and pepper.
Remove chicken from buttermilk, letting excess drip off, then press firmly into the panko mixture until fully coated.
Preheat the air fryer to 375°F and lightly spray the basket with avocado oil.
Place the chicken in the basket, spray the top of the chicken with avocado oil, and air fry for 12-15 minutes, flipping halfway through, until golden and cooked to 165°F.
While chicken cooks, whisk together the Greek yogurt and Dijon mustard in a small bowl.
Toast the whole wheat bun in a dry pan until lightly browned.
Spread the yogurt sauce on the bottom bun, then layer with lettuce, the crispy chicken, and pickles before closing the sandwich.