01 - Place the chicken breasts between two sheets of plastic wrap and pound them to an even thickness of about 1/2 inch using a meat mallet or rolling pin.
02 - In a mixing bowl, whisk together the buttermilk, 1 teaspoon of salt, and 1/2 teaspoon of black pepper until combined.
03 - Submerge the pounded chicken breasts in the buttermilk mixture, cover, and refrigerate for at least 30 minutes or up to 4 hours for maximum tenderness.
04 - In a separate shallow bowl or dish, combine the all-purpose flour, cornstarch, smoked paprika, garlic powder, cayenne pepper if using, remaining 1 teaspoon of salt, and 1/2 teaspoon of black pepper. Mix thoroughly.
05 - Remove each chicken breast from the marinade, allowing excess buttermilk to drip off. Press each piece firmly into the seasoned flour mixture, coating both sides evenly and ensuring the coating adheres well.
06 - Pour vegetable oil into a large skillet or deep pan to a depth of about 1/2 inch. Heat over medium-high heat until the oil reaches 350°F. Use a kitchen thermometer to verify the temperature.
07 - Carefully place the coated chicken breasts into the hot oil, working in batches to avoid overcrowding the pan. Fry for 6 to 7 minutes per side until golden brown and the internal temperature reaches 165°F.
08 - Transfer the fried chicken breasts to a paper-towel-lined plate to drain excess oil. Allow the chicken to rest for 2 to 3 minutes before serving to let the juices redistribute.