Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness to ensure even cooking.
In a shallow bowl, soak the chicken in the buttermilk for at least 15 minutes to tenderize the meat.
In a separate shallow dish, whisk together the panko breadcrumbs, smoked paprika, garlic powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, allowing the excess to drip off, then dredge it thoroughly in the seasoned panko mixture, pressing to adhere.
Heat the avocado oil in a non-stick skillet over medium-high heat until the oil is shimmering.
Carefully place the chicken in the skillet and cook for 4 to 5 minutes per side until the crust is golden brown and the internal temperature reaches 165°F.
While the chicken rests for a minute, lightly toast the whole wheat bun in the same skillet or a toaster.
Assemble the sandwich by placing the crispy chicken on the bottom bun and layering with the romaine lettuce, tomato slices, and dill pickles.