Place the chicken breast between two pieces of parchment paper and pound to an even half-inch thickness.
Submerge the chicken in a bowl with the buttermilk and let it marinate for at least 15 minutes.
In a shallow dish, combine the whole wheat panko, flour, garlic powder, paprika, salt, and pepper.
Remove the chicken from the buttermilk, allowing excess to drip off, then press firmly into the breading mixture until fully coated.
Lightly coat the chicken with avocado oil and place in the air fryer at 375 degrees Fahrenheit for 12 to 15 minutes, flipping halfway through.
While the chicken cooks, whisk together the Greek yogurt and Dijon mustard in a small bowl.
Toast the whole wheat bun until golden and spread the yogurt-mustard sauce on the bottom half.
Layer the lettuce, crispy chicken, and pickles on the bun before serving.