In a deep-fryer or large Dutch oven, pour oil to a depth of 10cm. Heat over medium heat until a pea-size drop of batter floats but does not swell, about 175°C.
In a medium bowl, combine 280g of flour, the water, and the egg. Whisk lightly until ingredients are combined (batter will be slightly lumpy). In a shallow dish, pour remaining flour. Dredge prawns in flour and then dip into the batter. Fry in batches for 2 to 3 mins per batch, turning to brown on all sides.
Remove from oil and drain on a cooling rack over a baking sheet. Serve hot with tempura sauce.
For the tempura sauce: In a small saucepan, combine the water, soy sauce, sweet rice wine, and seafood stock. Bring to a boil over medium-high heat. Reduce the heat to medium-low and simmer for 10 to 15 mins, stirring frequently.
* Cook's Note: When placing prawns into oil, hold by tail and allow to fan out before starting to fry.