Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
Place the chicken in a shallow bowl and pour the buttermilk over it, ensuring it is fully coated; let marinate for 10 minutes.
In a separate small bowl, whisk together the whole wheat flour, smoked paprika, garlic powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing excess to drip off, then dredge thoroughly in the flour mixture until coated.
Lightly brush or spray the breaded chicken with avocado oil and place in an air fryer basket at 375°F for 12-15 minutes, flipping halfway through, until the exterior is crispy and the internal temperature reaches 165°F.
While the chicken cooks, whisk the Greek yogurt and Dijon mustard together in a small ramekin to create a clean-eating spread.
Toast the whole wheat bun lightly, spread the yogurt-mustard sauce on the bottom half, top with the crispy chicken and pickles, and close the sandwich.