Place the reserved shells and heads in a large frying pan and cook over high heat for 7 minutes or until the shells are golden and cooked. Add the water and simmer for 5 minutes, then remove from the heat and strain through a fine sieve.
Place the laksa paste in a saucepan over medium heat and cook, stirring, for 3 minutes. Add the prawn stock, coconut milk and fish sauce and simmer for 5 minutes, then add the prawns, snow peas and kaffir lime leaf. Cook for a further 3 minutes or until the prawns have changed colour and are tender.
Place the noodles in a bowl of boiling water and allow to stand for 2 minutes or until soft. Drain. To serve, place the noodles, bean sprouts, mint and coriander in bowls. Spoon over the laksa liquid, prawns and snow peas and top with the chilli. Serves 4.
* There are a few versions of this spicy soup that originated in Singapore, known there as laksa lemak and now so popular elsewhere it has modern classic status.
** Shrimp paste and chillies are two of the important ingredients that give laksa paste its distinctive flavour. The heat of different laksa pastes will vary. For the best flavour, buy a good-quality laksa paste, or make your own.
*** In Singapore, Vietnamese mint is known as laksa leaf.