Boil and peel 15 very clean, large potatoes. Rub them through a fine sieve into a butter churn, add 8 tablespoons very cold, thick sour cream, and mix thoroughly. Gradually add 1 teacup very hot water and continue to churn for about 5 minutes after the last drop of water has been added. Then, while still churning, gradually add cold water until butter forms. Next, wash the butter in cold water with ice, add a little salt, and serve.
If you do not have a butter churn, mix everything in a stoneware bowl or soup tureen, adding water in the same sequence as indicated above.