Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
Place the chicken in a bowl and cover with buttermilk, marinating in the refrigerator for at least 30 minutes.
In a shallow dish, whisk together the flour, smoked paprika, garlic powder, onion powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting excess drip off, and dredge thoroughly in the flour mixture.
Lightly spray the chicken on both sides with avocado oil and place in the air fryer basket.
Air fry at 375°F for 12-15 minutes, flipping halfway through, until the exterior is golden and the internal temperature reaches 165°F.
While the chicken cooks, stir together the Greek yogurt and Dijon mustard in a small bowl.
Toast the whole-grain bun until lightly browned and crisp.
Spread the yogurt sauce on the bun and assemble the sandwich with lettuce, tomato, crispy chicken, and pickles.