Scrub the potatoes thoroughly, add the water and salt, and boil them over low heat until they are tender.
Pass through a sieve to remove the solid pieces, and reserve the basic stock. Add the powder made from the smoked shrimp heads to taste.
The Full-Bodied Version
Cooking 100g (3½oz) of dried porcini mushrooms or edible field mushrooms along with the potatoes makes the stock even better. To add more umami, combine equal parts of potato water and tomato juice made by squeezing the insides of ripe tomatoes.