Prepare the Onion: Peel the onion and cut a small base. Make vertical cuts without slicing through. Soak in ice water for 30 minutes.
Mix Dry Ingredients: In a bowl, mix flour, cornstarch, salt, paprika, garlic powder, black pepper, and cayenne.
Prepare Wet Mixture: Whisk buttermilk and egg in another bowl until smooth.
Coat the Onion: Dip the onion into the wet mix, covering it fully. Then, coat evenly in the dry mix.
Heat the Oil: Heat oil to 375°F in a deep fryer or pot.
Fry the Onion: Lower the onion into the oil with tongs. Fry for 3-4 minutes until golden and crispy.
Drain and Serve: Remove and place on paper towels to drain oil.
Make the Sauce: Stir together all the ingredients in a bowl to make the sauce.
Serve Hot: Put the crispy onion on a plate and serve with sauce.