Method
For the batter, sift the cornflour and flour together into a medium bowl.
Make a well in the centre and add the egg and beer and whisk from the centre outwards, until just combined (it is okay if the batter is a little lumpy). Allow the batter to stand for a few minutes in a cool place.
Heat the oil in a deep heavy-bottomed pan until a breadcrumb sizzles and turns brown when dropped into it. (CAUTION: hot oil can be dangerous. Do not leave unattended.)
Place the flour onto a plate and dredge the prawns into it to cover well. Dip the prawns into the batter and carefully add to the hot oil to deep-fry in batches for 2-3 minutes, or until crisp and golden.
Carefully remove with a slotted spoon and drain on kitchen paper.
Serve in bowls with a bowl of soy sauce for dipping.