Place the chicken breast between two sheets of parchment paper and pound to an even half-inch thickness.
Place the chicken in a small bowl and pour the buttermilk over it, ensuring it is fully coated, and let sit for 10 minutes.
In a shallow dish, whisk together the whole wheat flour, garlic powder, smoked paprika, sea salt, and black pepper.
Remove the chicken from the buttermilk and dredge it through the flour mixture, pressing the coating firmly onto both sides.
Heat the avocado oil in a non-stick skillet over medium-high heat.
Add the chicken to the skillet and cook for 5 to 6 minutes per side until the exterior is golden brown and the internal temperature reaches 165 degrees Fahrenheit.
Lightly toast the whole wheat bun in the skillet or a toaster.
Spread the Greek yogurt on the bottom bun, then layer with the lettuce, tomato, and the crispy chicken fillet before closing the sandwich.