Preheat the oven to 250 degrees F.
In a mixing bowl, combine the buttermilk, hot sauce, sugar and Essence and stir to blend.
In a shallow dish combine the flour, baking powder, black pepper, cayenne pepper, and salt and stir to blend. Peel the onions and slice crosswise into 1/2-inch-thick slices. Carefully separate the slices into separate rings.
Position a wire rack over a baking sheet. Dip the rings into the buttermilk and shake off any excess liquid. Working with a few slices at a time, dredge the slices into the flour mixture, shake off any excess, then dip again in the buttermilk and again in the flour mixture. Shake off any excess and transfer to the wire rack. Repeat with remaining onion slices.
Position a second wire rack over a baking sheet and place in the warmed oven. Fill a large, heavy Dutch oven with at least 2 inches of oil and heat to a temperature of 360 degrees F. (Or, alternatively, preheat a deep-fryer to 360 degrees F.) Fry the onion rings in batches, adding only as many rings as will fit without touching in 1 layer, until golden brown and floating on the surface of the oil, 2 to 3 minutes. Remove, using tongs, and transfer to a paper-lined plate to drain briefly, then transfer to the rack in the warm oven while you finish frying the remaining onion rings.
Serve onion rings hot.