Rinse shrimp under cold water, pat dry thoroughly with paper towels. Make 3 shallow diagonal cuts in the underside (belly) of each shrimp and gently straighten the shrimp by pressing along the back to avoid curling when fried. Season lightly with 1 tsp kosher salt and 1/4 tsp black pepper. Dust each shrimp lightly with 2 tablespoons cornstarch, shaking off excess; set on a tray and chill for 10 minutes while preparing batter and oil.

Tempura Batter

Chill liquids and dry mix

Place a metal or glass bowl in the freezer for 5 minutes to chill. Measure out 1 cup ice-cold sparkling water and add 4 small ice cubes just before mixing. In a separate bowl, whisk together 3 tablespoons cake flour, 3/4 cup all-purpose flour, 2 tablespoons rice flour (or cornstarch), 1/4 tsp baking soda (optional), and 1/4 tsp salt.

Mix batter

Lightly beat 1 cold large egg and combine with the ice-cold sparkling water. Pour the liquid into the chilled bowl with the dry ingredients and use chopsticks or a fork to mix with quick, light strokes — do not overmix; a few small lumps are okay. Keep batter cold by setting the bowl over another bowl filled with ice or by returning briefly to the fridge. The batter should be thin, slightly viscous, and cold; if it’s too thick, add 1–2 tablespoons more cold sparkling water.

Frying

Heat oil

Pour about 4 cups of neutral oil into a heavy-bottomed pot or deep skillet, leaving at least 2 inches of headspace. Heat oil over medium-high heat to 350–360°F (175–182°C). If you don't have a thermometer, test by dropping a small pinch of batter into the oil — it should sizzle and rise immediately without burning.

Batter and fry shrimp

One at a time, lightly dust chilled shrimp again if necessary, then dip each shrimp into the cold batter, allowing excess to drip off. Carefully lower shrimp into the hot oil, frying in batches of 4–6 so the oil temperature remains steady. Fry until the batter is pale golden and crisp, about 2–3 minutes per side (total ~3–4 minutes depending on size). Turn once if needed. Use a spider or slotted spoon to remove shrimp and drain on a wire rack set over a sheet pan or on paper towels. Keep finished shrimp warm in a low oven (200°F / 95°C) if frying in multiple batches.

Finish

If desired, squeeze a little lemon over the fried tempura shrimp and sprinkle a pinch of salt. Allow oil to cool, then strain and store or discard according to use.

Cold Noodle Bowl (to serve)

Prepare cold noodles: cook soba or somen according to package instructions, rinse thoroughly under cold running water until cool, then chill in the fridge. Divide 4 oz chilled noodles into bowls. Arrange 4–5 tempura shrimp on top of each bowl (for 20 shrimp this serves 4). Garnish with julienned cucumber (1/4 cup total), sliced scallions (2 tbsp), and 1 tbsp shredded nori or toasted sesame seeds. Serve with 3/4 cup cold mentsuyu or soy-dashi dipping sauce on the side, and lemon wedges if desired. To eat: dip hot crisp shrimp into the cold sauce and enjoy with chilled noodles.