In a large stock pot combine dried beans, water, and salt. Bring to a boil, reduce heat, cover and cook for 3 hours. When the beans are soft heat a large frying pan over medium-high heat. Add butter and let it melt. Scoop out beans and some of the broth into the frying pan. Using a potato masher, mash the beans and continue to add more beans and mash until all the beans have been added. Add more bean broth as needed to give a smooth consistency.