To make the sauce, put all the ingredients and 3 tbsp of water into a small pan. Bring to a simmer, stirring, and cook for 2 minutes or until the sugar has dissolved. Cool.
Pour just-boiled water over the noodles, cover and leave until just softened, then drain and rinse under cold water, then drain again.
Heat the oil in a wok or large frying pan. Fry the shallots for 2-3 minutes or until soft and golden, then add the prawns and cook for another 2 minutes until pink. Push the prawns and shallots to one side then add the egg, stirring until it has scrambled, then add the noodles and toss for 2 minutes. Add the sauce, chilli powder, bean sprouts and chives, and cook for 30 seconds.
Divide between two plates and scatter with peanuts, extra chives and serve with lime wedges to squeeze over.