Place the chicken breast between two sheets of plastic wrap and pound to an even half-inch thickness.
Submerge the chicken in a bowl with the buttermilk and let marinate for at least 15 minutes to tenderize.
In a shallow dish, whisk together the whole wheat flour, cornstarch, sea salt, black pepper, garlic powder, and paprika.
Remove the chicken from the buttermilk, allowing the excess to drip off, then dredge thoroughly in the flour mixture, pressing to ensure a thick coating.
Lightly mist both sides of the coated chicken with 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 and crispy.
Lightly toast the whole wheat bun and assemble the sandwich by layering the crispy chicken, romaine lettuce, and tomato slices.