Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
In a shallow bowl, combine the buttermilk, garlic powder, onion powder, paprika, salt, and pepper.
Submerge the chicken in the buttermilk mixture and marinate for at least 15 minutes.
Place the panko breadcrumbs in a separate shallow dish and dredge the marinated chicken until fully coated, pressing firmly to adhere.
Preheat the air fryer to 375°F and lightly spray the basket with avocado oil.
Place the chicken in the basket, spray the top with oil, and air fry 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 in a small bowl.
Lightly toast the brioche bun in the air fryer or a pan for 1 minute.
Spread the yogurt sauce on both sides of the bun, then assemble the sandwich with lettuce, tomato, pickles, and the crispy chicken.