Line 8 or 9 inch square pan with foil; butter foil.
In large heavy saucepan stir together first three ingredients; stir in milk, with a wooden spoon*.
Cook over medium heat, stirring constantly, until mixture comes to a full rolling boil.
Boil without stirring, to 234 degrees F on a candy thermometer (or until syrup, when dropped in very cold water forms a soft ball which flattens when removed from water). Bulb of candy thermometer should not rest on bottom of pan. (This can take 20-30 minutes).
Remove from heat. Add butter & vanilla.
DO NOT STIR! Cool at room temperature to 110 degrees F (lukewarm). (This can take 2 to 2-1/2 hours).
Beat with wooden spoon until fudge thickens & loses some of its gloss. (This can take 15-20 minutes. It really works best if you have someone to 'tag-team' with.) It starts to look more like frosting than a thick syrup when it is ready.
Quickly spread into prepared pan; cool.
Cut into squares.
Store wrapped loosely in foil in the refrigerator.
*it is very important not to use a wire whisk or the fudge will not set up. Also just stir gently, even though the cocoa will stay floating on top, it will mix in as the mixture heats up.
This recipe is from the back of a old Hershey Chocolate can