Line an 8 or 9 square pan with foil; butter foil.
In a large heavy saucepan stir together first three ingredients (sugar, cocoa & salt); stir in milk, with a wooden spoon**** 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.
Cook over medium heat, stirring constantly, until mixture comes to a full rolling boil.
Boil without stirring to 234º 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 and vanilla.
DO NOT STIR! Cool at room temperature to 110º (lukewarm this can take 2 to 2 1/2 hours).
Beat with wooden spoon until fudge thickens and 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.
*Alternately, when you take it off the stove to cool (or even quicker, put the pan in the sink with some cool water), begin to beat it until it turns
you can also add 2 cups of chopped pecans!
Quickly spread into prepared pan; cool.
Cut into squares.
Store loosely wrapped in foil in the refrigerator.
This recipe is from the back of an old Hershey Chocolate can.