Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness using a meat mallet or heavy skillet.
Submerge the chicken in a small bowl with the buttermilk and let it marinate for at least 15 minutes to tenderize the meat.
In a shallow dish, whisk together the whole wheat flour, smoked paprika, garlic powder, sea salt, and black pepper until well combined.
Remove the chicken from the buttermilk, letting any excess liquid drip off, then dredge thoroughly in the seasoned flour mixture until fully coated.
Preheat your air fryer to 375°F and lightly coat the air fryer basket with half of the avocado oil.
Place the chicken in the basket, drizzle or spray the remaining avocado oil on top, and cook for 12-15 minutes, flipping halfway, until the exterior is golden and crispy.
While the chicken cooks, lightly toast the whole wheat bun in a dry pan over medium heat for 1-2 minutes.
Assemble the sandwich by placing the crisp chicken on the bottom bun, followed by the butter lettuce and tomato slices, then top with the remaining bun half.