Peel potatoes and cut into quarters. Cook, covered, in simmering salted water for 15 to 20 minutes or until soft but not mushy. Drain potatoes and dry them by shaking pan over the heat. Rub the potatoes through a fine sieve or beat them with a wooden spoon. Beat in egg and butter. Drop the mixture, a teaspoonful at a time, into 375*F deep fat or oil. Fry the Bunuelos until golden brown; drain on absorbent paper. Serve very hot.