Put the potatoes and salt in a large pot and add enough water to cover the potatoes by about 3 inches. Bring to a 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 will in a colander.
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Set a potato ricer ove the pot and pass the potatoes through in batches. (Or return potatoes to the pot ad 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.
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