Place the chicken breast between two sheets of parchment paper and pound with a meat mallet to an even half-inch thickness.
In a shallow bowl, submerge the chicken in the buttermilk and let it marinate for at least 15 minutes to tenderize the meat.
In a separate shallow dish, whisk together the whole wheat flour, cornstarch, garlic powder, onion powder, paprika, salt, and pepper.
Remove the chicken from the buttermilk, allowing the excess to drip off, then dredge it thoroughly in the flour mixture until fully coated.
Heat the avocado oil in a large 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 exterior is golden brown and the internal temperature reaches 165°F.
While the chicken rests for a minute, lightly toast the whole grain bun in the same skillet or a toaster.
Assemble the sandwich by placing the romaine lettuce on the bottom bun, followed by the crispy chicken and the pickle chips.