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 kosher salt, and 1/2 teaspoon 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), 1 teaspoon kosher salt, and 1/2 teaspoon black pepper. Whisk until evenly blended.
05 - Remove each chicken breast from the marinade, allowing excess buttermilk to drain off. Press each piece firmly into the flour mixture, ensuring an even, well-adhered coating on all sides.
06 - Pour vegetable oil into a large skillet or deep pan to a depth of about 1/2 inch. Heat over medium-high heat to 350°F, using a kitchen thermometer to verify the temperature.
07 - Carefully place the coated chicken breasts into the hot oil in batches, being careful not to overcrowd 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. Let rest for 2 to 3 minutes before serving to allow the juices to redistribute.