Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
Place the chicken in a shallow bowl and pour the buttermilk over it, ensuring it is fully coated; marinate for at least 15 minutes.
In a small plate, whisk together the whole wheat flour, smoked paprika, garlic powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting any excess drip off, then dredge it through the flour mixture until evenly coated.
Lightly brush or spray the chicken with avocado oil and place it in the air fryer at 375°F for 12-15 minutes, flipping halfway through, until golden and cooked through.
While the chicken cooks, whisk together the Greek yogurt and Dijon mustard in a small bowl to create the sauce.
Toast the whole grain bun until lightly browned.
Assemble the sandwich by spreading the yogurt sauce on the bun, then adding the lettuce, crispy chicken, and pickles.