Add the coconut milk, salt and tamarind with 100ml/3½fl oz freshly boiled water. Make a couple of little incisions in each of the 3 whole finger chillies with the point of a small sharp knife and add to the pan. Turn the heat up to bring to a near boil, then reduce the heat and simmer gently for about 7 minutes, stirring frequently, until the sauce has cooked and reduced to a thick golden soup.