Place the chicken breast between two sheets of parchment paper and pound with a meat mallet to an even half-inch thickness.
Place the chicken in a shallow bowl and pour the buttermilk over it, ensuring it is fully submerged, and marinate for 15 minutes.
In a separate shallow dish, whisk together the cassava flour, smoked paprika, garlic powder, onion powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing excess to drip off, then dredge in the flour mixture, pressing firmly to create a thick coating.
Heat the avocado oil in a cast-iron skillet over medium-high heat until shimmering.
Carefully place the chicken in the skillet and cook for 4 to 5 minutes per side until the coating is deeply golden and the internal temperature reaches 165°F.
Remove the chicken from the pan and let it rest on a wire rack while you lightly toast the sprouted grain bun in the remaining oil.
Spread the avocado oil mayonnaise on the bottom bun, then layer with the romaine lettuce, tomato slice, crispy chicken, and dill pickles before topping with the other half of the bun.