Pour the oil in a large saucepan or wok to about 2 inches deep and heat to 360 degrees F.
In a large bowl, beat together the water and egg yolk until frothy. Whisk in the flour and cornstarch until just combined, but not completely smooth, no more than a minute.
Dip a pepper into the batter to coat completely, then place in the heated oil. Repeat with more peppers, being careful not to crowd the pan. Fry until golden on all sides, 3-5 minutes, then transfer to a towel-lined plate or rack. Repeat with remaining peppers, letting the oil get back to 360 degrees F between batches.
Serve immediately with tentsuyu or desired dipping sauce.