Peel and cut onions into slices, about 1/4-in. to 1/2-in. thick. Separate the slices into rings.
In a mixing bowl, whisk together the buttermilk and hot sauce. In another bowl, whisk together the flour, cornstarch, salt, pepper and baking powder. Combine the wet and dry ingredients together. (If batter is too thick, add a little water to thin.)
Using a deep fryer, add oil to maximum fill line in bowl. Preheat the oil to 375ºF.
Dip each ring into batter and then shake off extra batter. Place several onion rings into the deep fryer basket and carefully lower it into the preheated oil.
Cook onions about 1 1/2 - 2 minutes, or until the rings are golden brown. Use metal tongs to turn the rings during the cooking process.
Carefully lift the basket out of oil and lock it onto resting hooks. Allow onions to drain and then transfer them to a wire rack with a pan underneath.
Repeat the process with the remaining onions. Sprinkle the rings with salt and serve.