In a large pot, combine 1/4 lb butter (margarine) with 1 can condensed milk and two cups of sugar.
Bring to a slow boil, STIRRING ALL THE TIME! It just takes seconds for the mixture to start burning if you do not stir it.
After a few minutes you will see the mixture get caramel like in color and thicker. When you put a few drops on a cold plate, it will get fairly hard and fudge-like.
At this point you can turn the heat off and vigorously beat the mixture for two minutes (I use a wooden spoon throughout).
Pour the mixture out into a glass plate that have been sprayed or smeared with butter. Let is sit for ten minutes and then cut it in squares. Let it cool completely so that it is hard and see how long it lasts…!