In a pot, boil a little water and cook prawns until they turn pink. Remove, and when cool shell the prawns and reserve the water.
In a food processor, add shallots, chillies, shrimp paste, dried shrimp and blend to a paste. Add the powdered spices and mix.
Heat oil in a large pot and add the paste and lemongrass and cook for a minute. Add the prawn water and coconut milk while stirring, bring to the boil. Take of the heat, season with salt and pepper and add the sugar.
In another pot cook the wide rice noodles until soft. Drain and divide the noodles among 10 bowls. Add soaked glass noodles and bean sprouts. Garnish with prawns, fishcake, cucumber and Vietnamese mint. Pour over the broth and serve.