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 let marinate for at least 20 minutes to tenderize.
In another shallow dish, whisk together the whole wheat flour, garlic powder, onion powder, smoked paprika, salt, and pepper.
Remove the chicken from the buttermilk, letting the excess drip off, then dredge thoroughly in the flour mixture until fully coated.
Lightly coat the chicken with avocado oil and place in an air fryer at 375°F for 12-15 minutes, flipping halfway, until golden and crispy.
While the chicken cooks, whisk the Greek yogurt and Dijon mustard together in a small bowl to create a tangy spread.
Lightly toast the brioche bun until warm and fragrant.
Assemble the sandwich by spreading the yogurt-mustard sauce on the bun, then layering the lettuce, crispy chicken, and dill pickles.