Heat oil according in deep fryer, following manufacturer's instructions. Temperature should be 350-360 degrees F.
Mix buttermilk, egg, and hot sauce (I use 1 t., but you use more or less to taste) in a bowl. Place onion rings into bowl of buttermilk mixture. Let them sit in liquid while you prepare dry ingredients.
Whisk flour, cayenne, 1 t. salt, and pepper in a large plastic bag. Lift onion rings, letting them drip, then place into bag of dry mixture. Toss to coat.
Deep fry onions in batches until light brown, about 3 minutes, depending on fryer. The more onions you put in, the more heat will be lost, taking a little more time.
Place onion rings on a paper lined plate to drain. Immediately sprinkle salt (I prefer sea salt at this point) and serve.