If using frozen prawns, defrost thoroughly. Peel the prawns and discard the shells. Using a small sharp knife, remove the fine digestive cord. Wash the prawns in cold water, rinse well and pat dry with kitchen paper.
In a wok, bring the stock to simmering point. Add the chillies, salt, pepper, fish sauce or soy sauce, lime zest and lime juice. If you like it spicy, add the chilli powder. Simmer for another 3 Minutes.
Add the prawns, cover the wok and remove from the heat. Allow to stand, covered, for 10 Minutes.
Finally, stir in the coriander.
Ladle into a large soup tureen or individual bowls and serve immediately.