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 it marinate for 10 minutes.
In a separate shallow dish, whisk together the flour, panko, garlic powder, onion powder, smoked paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting excess drip off, then dredge thoroughly in the flour mixture, pressing to adhere.
Heat the avocado oil in a non-stick skillet over medium-high heat.
Cook the chicken for 5-6 minutes per side until the crust is golden brown and the internal temperature reaches 165 degrees Fahrenheit.
Lightly toast the whole grain bun in the same skillet or a toaster.
Assemble the sandwich by placing the lettuce on the bottom bun, followed by the crispy chicken and dill pickles.