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