Put the potatoes and salt in a large pot and add enough water to cover the potatoes by about 3 inches. Bring to boil over high heat, then reduce the heat and simmer until the potatoes are tender when pierced with a fork, 15 to 20 minutes. (Or steam them for about 30 minutes.) Drain well in colander.
Set a potato ricer over the pot and pass the potatoes through in batches. (Or return potatoes to the pot and mash with a potato masher.) Add the cauliflower puree, margarine, and buttermilk, and beat with a large spoon until the potatoes are smooth and creamy.