Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
Submerge the chicken in the buttermilk in a shallow dish and marinate for at least 20 minutes to tenderize.
In a small bowl, whisk together the whole wheat flour, smoked paprika, garlic powder, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting excess drip off, and dredge in the flour mixture until evenly coated.
Heat the avocado oil in a cast-iron skillet over medium-high heat until shimmering.
Carefully place the chicken in the skillet and sear for 5 to 6 minutes per side until the crust is golden and the internal temperature reaches 165 degrees.
While the chicken cooks, whisk together the Greek yogurt and Dijon mustard to create a clean sandwich spread.
Lightly toast the whole wheat bun in the skillet for the last minute of cooking.
Assemble the sandwich by spreading the yogurt sauce on the bun and layering with the butter lettuce, crispy chicken, and pickles.