Pound the Chicken Fillets: Place one chicken breast inside a resealable plastic bag on a cutting board. Strike the thickest part with a meat mallet, working outward from the center in firm, even strokes, until the fillet reaches a uniform 1.5cm / 0.6in thickness. Repeat with the remaining 3 breasts. Even thickness prevents the edges from drying out before the center reaches a safe internal temperature.
Brine in Buttermilk: Whisk together the buttermilk and hot sauce in a medium bowl until fully combined. Submerge all 4 pounded fillets in the buttermilk mixture, pressing them down so each fillet is fully coated. Cover the bowl with plastic wrap and refrigerate for 30 minutes. The lactic acid in buttermilk begins to tenderize the muscle fibers, and the hot sauce adds a mild underlying heat to the finished sandwich.
Build the Dredging Mixture: Combine the all-purpose flour, cornstarch, 1 teaspoon kosher salt, garlic powder, smoked paprika, onion powder, black pepper, and cayenne pepper in a wide, shallow dish. Whisk the dry ingredients together for 30 seconds until evenly distributed. The cornstarch raises the starch concentration in the coating, which converts to a more brittle, glass-like crust during frying.
Double-Dredge Each Fillet: Lift one fillet from the buttermilk and allow the excess to drip off for 5 seconds. Lay the fillet in the flour mixture and press firmly on both sides, working the coating into any surface folds. Return the fillet to the buttermilk for 3 seconds, then return it to the flour for a second press-and-coat. Set the coated fillet on a wire rack. Repeat with the remaining 3 fillets. Resting the coated fillets on the rack for 5 minutes before frying allows the coating to hydrate and bond to the surface.
Heat the Frying Oil: Pour the neutral cooking oil into a 12-inch cast iron skillet and set it over medium-high heat for 3 minutes until the oil reaches 175°C / 347°F, confirmed with an instant-read thermometer. At this temperature, a pinch of flour dropped into the oil will sizzle immediately and rise to the surface.
Fry the Chicken Fillets: Using tongs, lower 2 coated fillets gently into the hot oil, laying them away from you to avoid splatter. Fry for 5 minutes on the first side without pressing or moving them until the coating is deep golden brown and the crust releases cleanly from the skillet. Flip each fillet once with tongs and fry for 4 minutes on the second side until the internal temperature reaches 74°C / 165°F, confirmed with an instant-read thermometer inserted into the thickest point. Transfer the cooked fillets to the wire rack. Allow the oil to return to 175°C / 347°F for 1 minute before frying the second batch.
Fry the Second Batch: Lower the remaining 2 coated fillets into the oil using tongs. Fry for 5 minutes on the first side until the crust is deep golden brown and fully set. Flip and fry for 4 minutes on the second side until the internal temperature reaches 74°C / 165°F at the thickest point. Transfer to the wire rack and season immediately with the remaining 0.5 teaspoon kosher salt while the crust is still hot.
Make the Sandwich Sauce: Stir together the mayonnaise and dill pickle brine in a small bowl until the mixture is smooth and uniform. The pickle brine adds acidity and salt to the sauce, which cuts through the richness of the fried coating. Spread 1 tablespoon of sauce on the cut side of each toasted bun top.
Assemble Each Sandwich: Place one lettuce leaf on each toasted bun bottom, followed by one fried chicken fillet, 2 tomato rounds, and 1 dill pickle slice. Press the sauced bun top firmly onto the stack. Serve each chicken sandwich immediately while the crust retains its texture.
