Preheat the oven to 450º. Add cornflakes and breadcrumbs to a food processor and grind until you get fine crumbs. Remove it in a bowl. In another bowl, whisk buttermilk, egg, cayenne, flour, salt, and pepper. Dip the onion rings in the egg mixture, drip off the excess mixture, and dredge them through the cornflake mixture. Place the rings on a wire rack.