Place the chicken breast between two sheets of parchment paper and pound with a mallet to an even 1/2-inch thickness.
In a small bowl, submerge the chicken in the buttermilk and let marinate for at least 15 minutes.
In a shallow dish, whisk together the almond flour, arrowroot powder, smoked paprika, garlic powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing excess to drip off, then dredge in the flour mixture, pressing firmly to ensure a thick coating.
Lightly coat the breaded chicken on both sides with avocado oil spray.
Place the chicken in the air fryer basket and cook at 400°F for 12 to 15 minutes, flipping halfway through, until the exterior is golden and the internal temperature reaches 165°F.
While the chicken cooks, lightly toast the whole grain bun in a dry pan or toaster.
Assemble the sandwich by spreading Dijon mustard on the bottom bun, adding the crispy chicken, and topping with dill pickles.