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 submerged; marinate for at least 20 minutes in the refrigerator.
In a separate small bowl, whisk together the whole wheat flour, smoked paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk, letting excess drip off, then dredge it through the flour mixture until evenly coated.
Heat the olive oil in a non-stick skillet over medium-high heat. Once hot, add the chicken and cook for 5-6 minutes per side until golden brown and cooked through.
While the chicken cooks, whisk together the Greek yogurt and Dijon mustard in a small ramekin to create the sauce.
Lightly toast the whole grain bun in a toaster or on the edge of the skillet.
Assemble the sandwich by spreading the yogurt sauce on the bottom bun, followed by the butter lettuce, the cooked chicken, and the dill pickles before topping with the other bun half.