Place the chicken breast between two sheets of parchment paper and lightly pound to an even thickness.
Submerge the chicken in a bowl with the buttermilk and let marinate for at least 30 minutes in the refrigerator.
In a shallow dish, whisk together the whole wheat flour, smoked paprika, garlic powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing excess to drip off, then dredge thoroughly in the flour mixture until well coated.
Lightly coat the chicken on both sides with the avocado oil spray.
Place the chicken in an air fryer basket and cook at 375°F for 12-15 minutes, flipping halfway through, until the exterior is golden brown and crispy.
While the chicken cooks, whisk the Greek yogurt and Dijon mustard together in a small bowl to create a healthy spread.
Toast the whole grain bun until lightly browned.
Spread the yogurt-mustard sauce on the bottom bun, then layer with lettuce, tomato, the crispy chicken, and pickles before topping with the other bun half.