This dish gets a lot of its flavour from belacan - salted, fermented shrimp paste. It has a very strong smell that some people find unpleasant, but it's essential to many Asian cuisines. There are different types of belacan: some are like a soft, spoonable paste that is sold in jars, but for this dish, buy the dried shrimp paste that comes in a small, hard block. When mixed with other ingredients, the pungency and strong flavour are muted, and the belacan adds a rich umami to the dish.
Cloud ear mushrooms, also called cloud ear fungus, are most easily available dried. They're small and light, but grow enormously when soaked. You can substitute the similar wood ear mushrooms.
Serve these noodles with fresh lime juice and sambal belacan - chilli shrimp paste, which you can either buy (it comes in jars) or make yourself.
