Cook the rice noodles according to the pack instructions and put to one side.
In a small bowl mix the tamarind paste with the boiling water, Thai fish sauce and sugar. Put to one side.
Heat the oil in a large wok or frying pan and cook the shallots and chillies for 30 seconds then add the prawns and cook for 2-4 minutes until they have turned pink.
Add the noodles, spring onions, beansprouts and sweet and sour tamarind paste and cook for a further minute. Add a squeeze of the lime juice and stir together.
Serve garnished with the cashew nuts and coriander.