Pound the chicken breasts to 3/4-inch thickness, submerge in the 1 cup buttermilk mixed with 1 tsp hot sauce, and refrigerate for at least 30 minutes.

Whisk together the 1 cup flour, 2 tbsp cornstarch, 1 tsp garlic powder, 1 tsp onion powder, 1 tsp smoked paprika, 1 tsp salt, 1/2 tsp black pepper, and 1/2 tsp cayenne in a wide shallow bowl.

Double-dredge each fillet: flour, back in buttermilk, flour again, pressing firmly each time. Rest on a wire rack for 5 minutes.

Heat 2 inches of neutral oil in a heavy pan to 350°F (175°C). Fry each fillet 5 to 6 minutes per side until the crust is deep amber and the internal temp reaches 165°F (74°C). Transfer to a wire rack.

Toast the 2 brioche buns cut-side down in a dry skillet over medium heat for 1 to 2 minutes. Spread 2 tbsp mayonnaise on each bottom bun, add 1/2 cup shredded iceberg lettuce, place the hot fillet on top, and close the sandwich.