Place the chicken breast between two pieces of parchment paper and pound to an even half-inch thickness.
Place the chicken in a shallow bowl and cover with buttermilk; marinate in the refrigerator for at least 20 minutes.
In a separate shallow dish, whisk together the whole wheat flour, smoked paprika, garlic powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing excess to drip off, and dredge thoroughly in the flour mixture, pressing to ensure a thick coating.
Lightly spray both sides of the breaded chicken with avocado oil.
Place in the air fryer basket and cook at 375°F for 12-15 minutes, flipping halfway through, until the exterior is golden brown and the internal temperature reaches 165°F.
During the last 2 minutes of cooking, place the split whole wheat bun in the air fryer to toast.
Assemble the sandwich by placing the crispy chicken on the toasted bun and layering with romaine lettuce, tomato slices, and pickles.