Whisk together the cornmeal, flour, sugar, salt, and baking powder in a large bowl.
Whisk together the buttermilk, water, butter, hot sauce, and onion in a medium bowl.
Add the buttermilk mixture to the cornmeal mixture and mix with a spoon or rubber spatula until well combined.
Let the mixture rest for 10 minutes.
Meanwhile, heat the oil to 375°F in a deep-fryer or deep skillet to at least 2 inches deep.
Transfer the hushpuppy mixture to a piping bag fitted with a large round hole tip.
Working in batches, pipe 2-inch lines of batter into the hot oil.
Fry until golden brown, about 2-3 minutes.
Transfer the hushpuppies to a paper towel-lined baking sheet or platter.
Repeat the cooking process with any remaining batter.
Serve immediately with butter, if desired.
