Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness for uniform cooking.
Submerge the chicken in the buttermilk in a shallow bowl, ensuring it is fully coated, and let it marinate for at least 20 minutes.
In a separate wide dish, whisk together the whole wheat flour, garlic powder, onion powder, smoked paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting excess drip off, then dredge thoroughly in the flour mixture until every nook is covered.
Lightly coat the breaded chicken on both sides with the avocado oil to ensure a golden, crunchy finish in the air fryer.
Place the chicken in the air fryer basket and cook at 375°F for 12 to 15 minutes, flipping halfway through until the internal temperature reaches 165°F.
Lightly toast the whole grain bun until the edges are golden brown and warm.
Assemble the sandwich by placing the crispy chicken on the bottom bun, topped with the fresh romaine leaf and zesty pickle slices.