In a large bowl, whisk together the cornmeal, flour, sugar, baking powder, and salt. In a medium bowl, whisk together the buttermilk, butter, hot sauce, onion, and ¼ cup of water. Using a spoon, stir the buttermilk mixture into the flour mixture. Set aside for 10 minutes, then use a silicone spatula to transfer the batter to a piping bag fitted with a ¾-inch round tip.
Into a large heavy pot fitted with a deep-fry thermometer, pour the oil to a depth of 2 inches and turn the heat to medium-high. When the temperature reads 375°F, working in batches, pipe 3-inch logs of batter into the oil. Fry, turning occasionally, until golden brown, 2–3 minutes total. Using a slotted spoon, transfer to a paper towel-lined plate. Season the hush puppies to taste with more salt, and serve immediately.