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 marinate for 15 minutes.
In a separate shallow dish, whisk together the flour, smoked paprika, garlic powder, onion powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting any excess drip off, then dredge it through the flour mixture, pressing firmly to create a thick crust.
Heat the avocado oil in a cast-iron or heavy-bottomed 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, or until the coating is golden brown and the internal temperature reaches 165°F.
While the chicken rests, lightly toast the whole wheat bun in the same skillet for about 1 minute.
Assemble the sandwich by spreading Dijon mustard on the bottom bun, followed by the lettuce, the crispy chicken, and the pickles.