Make the marinade: in a large bowl whisk buttermilk, salt, garlic powder. Add the chicken thighs, make sure the chicken is covered, cover with plastic wrap and refrigerate for at least 2 hours or overnight.
Prepare the breading: in a large shallow bowl mix flour, salt, smoked paprika, black pepper, garlic powder, onion powder. Add the marinated chicken to the flour mixture and roll until fully coated. Shake off the excess. Repeat until you finish coating all chicken pieces.
Frying: fill a pot with vegetable oil. Heat oil to 350°F-375°F (175°C-190°C).
Place the breaded chicken into the hot oil, 2-3 pieces at the time. Fry for about 6-7 minutes, until the coating is golden brown and the chicken reaches an internal temperature of 165°F (75°C).
Transfer to a wire rack or paper towels to drain. Let sit for 10 minutes before serving.
Meanwhile make the sauce: in a bowl, whisk all sauce ingredients.
Assembly: cut the buns in half and toast them. Spread the sauce on both sides of the bun. Lay few pieces of pickles, lettuce and other toppings that you like. Lay the chicken, then put the other half of the bun on top.