Place the chicken breast between two sheets of parchment paper and pound with a meat mallet until it is an even half-inch thickness.
Submerge the chicken in the buttermilk in a shallow bowl and marinate in the refrigerator for at least 30 minutes.
In a separate small bowl, whisk together the whole wheat flour, garlic powder, onion powder, paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing the excess to drip off, then dredge it thoroughly in the seasoned flour mixture until fully coated.
Lightly brush or spray both sides of the coated chicken with the olive oil to ensure a crispy exterior.
Place the chicken in an air fryer basket and cook at 375°F for 12 to 15 minutes, flipping halfway through, until the internal temperature reaches 165°F.
While the chicken cooks, lightly toast the whole wheat bun in a dry pan or toaster until warm and slightly firm.
Assemble the sandwich by placing the crispy chicken on the bottom bun, followed by the romaine lettuce and tomato slices, then top with the remaining bun half.