Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
In a shallow bowl, whisk together the buttermilk, sea salt, black pepper, smoked paprika, and garlic powder.
Add the chicken to the buttermilk mixture, ensuring it is fully coated, and let marinate for at least 10 minutes.
Place the arrowroot powder in a separate shallow dish and dredge the marinated chicken, pressing the powder into the meat to create a light crust.
Heat the avocado oil in a non-stick skillet over medium-high heat.
Add the chicken to the skillet and cook for 5-6 minutes per side, or until the exterior is golden brown and the internal temperature reaches 165°F.
While the chicken cooks, whisk together the Greek yogurt and Dijon mustard in a small bowl to create a healthy spread.
Lightly toast the whole grain bun in the skillet or a toaster.
Assemble the sandwich by spreading the yogurt-mustard sauce on both sides of the bun, then layering the lettuce, tomato, crispy chicken, and pickle chips.
Serve immediately while the chicken is hot and the crust is crisp.