Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
In a shallow bowl, soak the chicken in buttermilk for at least 15 minutes to tenderize.
In a separate shallow dish, whisk together the flour, cornstarch, smoked paprika, garlic powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing excess to drip off, and dredge thoroughly in the flour mixture until well coated.
Heat the avocado oil in a non-stick skillet over medium-high heat until shimmering.
Carefully place the chicken in the skillet and cook for 5-6 minutes per side until the crust is golden brown and the internal temperature reaches 165 degrees.
While the chicken rests, toast the whole wheat bun in the same skillet for about 1 minute until lightly browned.
In a small bowl, stir together the Greek yogurt and Dijon mustard to create a clean, tangy spread.
Assemble the sandwich by spreading the yogurt sauce on the toasted bun, followed by the lettuce, crispy chicken, and pickle slices.