Straighten the shrimp: Make 4 to 5 shallow diagonal slits across the belly of each shrimp (about ⅓ of the way through—do not cut through). Lay the shrimp belly-down on a cutting board. Press firmly along the back with your fingers until you feel small muscle fibers 'pop' or release. The shrimp should now stay completely flat. Scrape the tail fins with the back of your knife to squeeze out excess moisture, which prevents oil splattering. Thoroughly pat all shrimp dry with paper towels.

Heat the oil: Heat oil in a heavy pot or Dutch oven to 350°F–360°F (175°C–180°C). Maintaining this range prevents greasy, soggy tempura.

Mix the batter (at the very last second): Whisk the ice water and egg yolk together in a bowl. Add the flour, starch, and baking powder. Using chopsticks or a fork, stir with a stabbing motion for only 10 to 15 seconds. Visible dry flour pockets and lumps are essential. Do not whisk smooth.

Dredge, dip, and fry: Lightly dust each shrimp in dry starch, tapping off almost all excess (this gives the batter traction). Holding the shrimp by the tail, dip it into the cold batter, then immediately lower it into the hot oil. Fry 3 to 4 shrimp at a time to prevent oil temperature from dropping. Cook for 2 to 2½ minutes, turning once, until pale ivory-gold and rigid. Transfer to a wire cooling rack (paper towels trap steam and soften the crust). Skim out any stray batter flakes between batches.