Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
In a shallow bowl, soak the chicken in buttermilk for at least 15 minutes.
In a separate bowl, whisk together the whole wheat flour, cornstarch, garlic powder, onion powder, paprika, salt, and pepper.
Remove chicken from buttermilk, letting excess drip off, then dredge thoroughly in the flour mixture, pressing to adhere.
Lightly brush or spray the chicken with avocado oil and place in an air fryer at 375°F for 12-15 minutes, flipping halfway, until golden and cooked through.
Toast the whole wheat bun until lightly browned.
Assemble the sandwich by placing the crispy chicken on the bun and topping with lettuce, tomato, and pickles.