Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
Submerge the chicken in buttermilk in a shallow bowl and refrigerate for at least 30 minutes to tenderize.
In a separate bowl, whisk together the whole wheat flour, cornstarch, garlic powder, onion powder, paprika, salt, and pepper.
Remove the chicken from the buttermilk, letting excess drip off, then dredge thoroughly in the flour mixture until fully coated.
Lightly coat the chicken with avocado oil spray and place in the air fryer basket at 375°F.
Air fry for 12 to 15 minutes, flipping halfway through, until the exterior is golden brown and the internal temperature reaches 165°F.
Toast the whole grain bun until warm and assemble the sandwich with lettuce, tomato slices, pickles, and the crispy chicken.