Place chicken breast between parchment paper and pound to an even thickness.
In a small bowl, soak the chicken in buttermilk for at least 15 minutes.
In a separate shallow dish, combine panko, garlic powder, smoked paprika, salt, and pepper.
Remove chicken from buttermilk, letting excess drip off, and dredge thoroughly in the seasoned panko mixture.
Lightly coat the chicken with avocado oil and air-fry at 375°F for 12-15 minutes, flipping halfway, until golden and crispy.
While chicken cooks, whisk together Greek yogurt and Dijon mustard to create a zesty spread.
Toast the bun until warm and lightly browned.
Assemble the sandwich by spreading the yogurt sauce on the bun, then layering the lettuce, crispy chicken, and pickles.