Wash the shishito peppers and pat dry with a towel to remove all moisture. Prick each pepper a couple of times with a toothpick or sharp knife along the sides. This will help keep them from bursting.
Pour the oil in a large saucepan or wok to about 2 inches (5 centimeters) deep and heat to 360˚F (185˚C).
In a large bowl, beat together the water and egg yolk until frothy.
Whisk in the flour and cornstarch using chopsticks until just combined, but not completely smooth, no more than a minute.
Dip a pepper into the batter to coat completely, allow any excess to drip off, then place in the heated oil. Repeat with more peppers, taking care to not crowd the pan.
Cover the pan with a splatter guard.
If the batter is too thick when coating the peppers, mix in a little more ice cold water. If too thin and falling off, mix in a little more flour.
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˚F (185˚C) between batches.
Serve immediately with a sprinkling of salt or alongside desired dipping sauce.