Prepare the shrimp and all the vegetables. Store everything in the refrigerator until right before frying.
Make the tempura batter
Place the iced sparkling water into a large mixing bowl. Lightly beat the egg yolk and gradually pour into the iced water, stirring (preferably with chopsticks) and blending well. Add flours and baking powder all at once, stroke a few times with chopsticks until the ingredients are loosely combined. The batter should be runny and lumpy. Place the bowl of batter in an ice water bath to keep it cold while you are frying the tempura.
Start by frying the vegetable, cooking those with a delicate flavor first. (Each vegetable will add some flavor to the frying oil.) Dip each piece of vegetable in a shallow bowl of flour to lightly coat them and then dip them into the batter. Slide them into the hot oil, deep frying only 3 to 5 pieces at a time (depending on the size of your pan) so that the temperature of the oil does not drop below the recommended temperature.
Place the fried tempura pieces on a wire rack set in a baking sheet lined with paper towels so excess oil can drip off. Sprinkle the tempura with flaky sea salt, if desired. Keep the cooked tempura pieces in a 250°F (120°C) preheated oven while you cook the rest.
Fry the shrimp the same way, dredging in flour first, dipping into the batter second. Plunge the shrimp into the oil holding it by the tail so it staysstraight and doesn’t curl up.
SERVING: Serve the hot, crispy shrimp and vegetable tempura with Japanese soy sauce (shoyu) for dipping.
How to Prepare Shrimp for Tempura
Make sure to use large, 16-20 tiger shrimp.
Peel the shrimp, but keep the tail.
Place the shrimp belly side down on a cutting board, flattening it
Using a very sharp paring knife, make a shallow insertion along the back of the shrimp.
With the tip of the knife, pull out the grey-black vein from the back of the shrimp and discard.
Turn the shrimp around and make a second shallow insertion along the belly of the shrimp and remove and discard the whitish vein you'll find in there.
Make 4 to 5 very shallow, crosswise insertions long the belly of the shrimp. This will help keep the shrimp straight during the frying process.
Turn the shrimp back over to make it sit on its belly. Gently press it down—you'll hear the shrimp "crunch" a bit as you flatten it.
If desired, cut off the tip of the tail to make it perfectly straight.