Place the chicken between two sheets of wax paper or large resealable bag, one piece at a time and pound with a meat mallet or wooden rolling pin until ½-inch thick.
Place chicken in a large resealable bag. Add buttermilk and push around with your fingers to coat all sides. Refrigerate for 24 hours, turning occasionally.
In a shallow dish, combine the flour and spices with a spoon.
Pour about 1-inch of oil into a large cast iron pan. Heat until very hot, about 360 degrees.
Lift one piece of chicken at a time, without wiping off any buttermilk and dredge both sides in the seasoned flour. Lightly press with the chicken with your hands until completely coated. Gently shake off any excess flour and place into the hot oil.
Fry chicken until lightly golden brown and cooked all the way through. About 3-4 minutes per side. Use tongs to flip and remove the chicken from the hot oil. Transfer the chicken on paper towels.
Assemble the sandwiches and serve warm.
Make sure to use a digital thermometer to monitor the temperature of the cooking oil.