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 submerged; let it marinate for 15 minutes.
In a separate shallow dish, whisk together the arrowroot powder, smoked paprika, garlic powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting the excess drip off, then dredge it thoroughly in the arrowroot 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 to 6 minutes per side until the crust is golden-brown and the internal temperature reaches 165 degrees Fahrenheit.
While the chicken rests for 2 minutes, lightly toast the sprouted grain bun in the same skillet or a toaster.
Assemble the sandwich by placing the lettuce and tomato on the bottom bun, followed by the crispy chicken and dill pickles, then top with the remaining bun half.