Place the chicken breast in a small bowl and pour the buttermilk over it, ensuring it is fully submerged to marinate for at least 15 minutes.
In a separate shallow dish, whisk together the all-purpose flour, paprika, garlic powder, sea salt, and black pepper until well combined.
Remove the chicken from the buttermilk, letting any excess drip off, then dredge it thoroughly in the flour mixture until evenly coated.
Lightly brush or spray the coated chicken with avocado oil to ensure a golden finish when cooking.
Place the chicken in an air fryer basket and cook at 400°F for 12 to 15 minutes, flipping halfway through, until the internal temperature reaches 165°F.
While the chicken cooks, lightly toast the whole wheat bun in a dry pan or toaster until warm.
Assemble the sandwich by spreading yellow mustard on the bottom bun, followed by the romaine lettuce, the crispy chicken, and the dill pickles.