Place the chicken breast between two sheets of parchment paper and pound with a meat mallet to an even half-inch thickness.
Place the chicken in a shallow dish and pour the buttermilk over it, ensuring it is fully submerged; marinate in the refrigerator for at least 30 minutes.
In a separate shallow bowl, whisk together the whole wheat flour, cornstarch, garlic powder, onion powder, paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting the excess drip off, then dredge it through the flour mixture, pressing firmly to ensure an even coating.
Lightly brush or spray the coated chicken with avocado oil on both sides.
Place the chicken in an air fryer basket and cook at 375°F for 12 to 15 minutes, flipping halfway through, until the internal temperature reaches 165°F and the exterior is golden brown.
Lightly toast the whole grain bun, then assemble the sandwich by layering the lettuce, crispy chicken, and pickles.