Whisk buttermilk and hot sauce in a bowl and marinate chicken for 30 minutes at room temperature.
Combine flour, cornstarch, and 1 tablespoon salt in a shallow tray.
Dredge each chicken piece in the flour mixture, dip back into the buttermilk, and dredge a second time in the flour to ensure a thick crust.
Heat frying oil in a heavy-bottomed skillet or deep fryer to 350 degrees Fahrenheit.
Fry chicken pieces for 6 to 8 minutes per side until golden brown and the internal temperature reaches 165 degrees Fahrenheit.
Whisk 1/2 cup of the hot frying oil into a heatproof bowl containing cayenne, brown sugar, paprika, and garlic powder until a smooth paste forms.
Immediately brush or dip the hot fried chicken into the spicy oil mixture to coat thoroughly.
Toast brioche buns and assemble sandwiches by placing chicken on the bottom bun and topping with four pickle slices.