Place the beans in a large pot, add water to cover by about 3 inches, and bring to a boil. Lower the heat to maintain a gentle boil and cook, half-covered, until the beans are completely softened, about 3 hours. Remove from the heat, drain, and using a slotted spoon, transfer to a basket. Let stand, loosely covered in a basket topped with a cotton cloth, in a warm place (but not in direct sunlight) to ferment for 2½ to 3 days.
When you smell an agreeable, slightly sweet fermented odor, you’ll know you’re there. (If it smells bad or you see mold, discard the beans. I haven’t had this experience, but Mother Nature is sometimes fickle!)
You will have about 5 cups cooked beans. Working in batches, grind the beans to a smooth, thick paste in a food processor or a mortar. Add a little water as you grind the beans if you need to: I find I need to add a couple of tablespoons into the beans when using the processor. Transfer the paste to a large bowl.
Stir in the salt, allowing about ½ teaspoon salt per cup of paste; taste and add a little more salt if you wish. The paste will keep in the refrigerator, sealed in a glass jar, for about a week. Use it in cooking as a flavoring in place of shrimp paste; do not eat it raw.