Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
In a shallow bowl, submerge the chicken in buttermilk and let it marinate for at least 20 minutes in the refrigerator.
In a separate small bowl, 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, then dredge thoroughly in the flour mixture until fully coated.
Lightly brush or spray the coated chicken on both sides with avocado oil to ensure a golden finish.
Place the chicken in the air fryer basket and cook at 375 degrees Fahrenheit for 12 to 15 minutes, flipping halfway through, until the internal temperature reaches 165 degrees.
Lightly toast the whole grain bun, then assemble the sandwich by layering the romaine lettuce, crispy chicken, and dill pickles.