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 the buttermilk for at least 15 minutes to tenderize.
In a separate small bowl, whisk together the whole wheat flour, cornmeal, smoked paprika, garlic powder, salt, and pepper.
Remove the chicken from the buttermilk, letting any excess drip off, then dredge thoroughly in the flour mixture, pressing to adhere.
Lightly coat the chicken with avocado oil using a spray or brush.
Place the chicken in the air fryer at 375°F (190°C) for 12-15 minutes, flipping halfway through, until the internal temperature reaches 165°F and the exterior is golden brown.
While the chicken cooks, lightly toast the whole wheat bun in a dry pan or toaster.
Assemble the sandwich by placing the lettuce and tomato on the bottom bun, followed by the crispy chicken and the top bun.