Rinse the prawns under cold water, pat dry with paper towels. Make 3 shallow diagonal slits along the underside of each prawn (to the tail) and gently straighten them. Leave tails intact for presentation. Lightly season the prawns with 1/2 tsp salt and 1/2 tsp black pepper.

Dust prawns

Combine the 2 tbs all-purpose flour and 1 tbs rice flour (or extra cornstarch) in a shallow dish. Lightly dredge each prawn in this dry mixture, shaking off any excess. This helps the batter adhere.

Make ice-cold batter

In a mixing bowl, whisk the lightly beaten egg. Add 3 tbs all-purpose flour, 4 tbs cornstarch, and 1 tsp baking powder. Add 1 cup ice-cold sparkling water gradually and stir very briefly — do not overmix; a few lumps are fine. Drop in 2–3 ice cubes into the batter bowl to keep it cold while frying. The batter should be thin and slightly lumpy.

Frying

Heat oil

Add about 1.2 litres of vegetable oil to a deep fryer or a deep saucepan to a depth of about 6–8 cm. Heat the oil to 175–180°C (350–360°F). If you don't have a thermometer, test by dropping a small bit of batter: it should sizzle and rise to the surface immediately and brown in about 40–50 seconds.

Batter and fry prawns

Using tongs, hold a dusted prawn by the tail and dip the body into the cold tempura batter, letting excess drip off briefly. Carefully lower the prawn into the hot oil. Fry in small batches (3–4 prawns at a time) to avoid temperature drop. Fry until batter is crisp and pale golden, about 2.5–3 minutes, turning once if needed. Do not over-brown — tempura is meant to be light and airy. Maintain oil temperature between batches (reheat to 175–180°C).

Drain

Using a slotted spoon or spider, transfer fried prawns to a wire rack set over a baking tray or to paper towels to drain. Keep in a warm oven (about 90°C / 200°F) if frying multiple batches, to keep prawns crisp while you finish.

Garnish & Serve

Arrange the tempura king prawns on a serving platter. Sprinkle the chopped coriander, if using. Serve immediately with lemon wedges and sweet chilli sauce on the side for dipping.