01 - Place the chicken breasts between two sheets of plastic wrap and use a meat mallet to pound them to an even 1/2-inch thickness for uniform cooking.
02 - In a mixing bowl, whisk together the buttermilk, 1 teaspoon kosher salt, and 1/2 teaspoon black pepper until dissolved.
03 - Submerge the pounded chicken breasts in the buttermilk brine, cover, and refrigerate for at least 30 minutes or up to 4 hours for maximum tenderness.
04 - In a separate shallow bowl or baking 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 until evenly blended.
05 - Remove each chicken breast from the buttermilk, allowing excess liquid to drip off, then press firmly into the seasoned flour mixture on all sides to build a thick, even coating.
06 - Pour vegetable oil into a large skillet or deep pan to a depth of about 1/2 inch and heat to 350°F, verifying the temperature with a kitchen thermometer.
07 - Carefully lower the coated chicken breasts into the hot oil in batches without overcrowding the pan. Fry for 6 to 7 minutes per side until deep golden brown and the internal temperature reaches 165°F.
08 - Transfer the fried chicken to a paper-towel-lined plate and let rest for 2 to 3 minutes to allow juices to redistribute before serving.