Place the chicken breast between two sheets of parchment paper and pound with a meat mallet until it reaches an even half-inch thickness.
Place the chicken in a shallow bowl and pour the buttermilk over it, ensuring it is fully submerged, and marinate in the refrigerator for 30 minutes.
In a separate shallow dish, whisk together the whole wheat flour, cornmeal, garlic powder, smoked paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting any excess drip off, then dredge it in the flour mixture, pressing firmly on both sides to create a thick coating.
Lightly spray the chicken on both sides with olive oil and place in the air fryer basket at 375°F for 12 to 15 minutes, flipping halfway through, until golden brown.
Lightly toast the whole grain bun and assemble the sandwich by layering the bottom bun with lettuce, tomato, the crispy chicken, and the top bun.