Line an 8 inch square pan with buttered parchment paper.
In a medium heavy saucepan stir together sugar, cocoa and salt, then add the milk and stir with a wooden spoon (not a whisk or metal spoon).
Cook over medium heat stirring continuously until mixture starts to boil (a strong boil), approximately 15 minutes.
Turn heat to low and continue to cook without stirring until temperature reaches 234 °F (112C) Approximately 30 minutes.
Remove pot from heat, add the butter and vanilla. Do not stir. Cool to room temperature (110°F/43C), approximately 30 minutes. Then beat with a wooden spoon just until mixture begins to lose some of it's gloss (approximately 6-8 minutes).
Then spread quickly in prepared cake pan and let cool completely, even overnight (at room temperature). Cut into squares. Store in an airtight container at room temperature.
You can also freeze the fudge in a double bag, well wrapped. Freeze either in pieces or the whole slab. Can be frozen up to 3 months. Keeps in an airtight container for approximately 2 weeks or in the fridge for 3-4 weeks. Enjoy!
For High Altitude - to adjust the temperature to your altitude, deduct two degrees from the temperature required for every one thousand feet of altitude above sea level. For example, in the recipe that follows, if you live 5,000 feet above sea level, you would deduct 10 degrees from 234 degrees. https://www.chicagotribune.com/news/ct-xpm-1988-01-28-8803260034-story.html Helpful tips"Absolutely loved making this. I doubled the batch and the temp took a little over an hour to get there. Adjusting the temp to almost a medium on our ultra heat burner. Then I placed my pot by a cracked window to allow the cold air to help cool it down to 110. When it’s only 9° out this helped! Added almond slices and coconut flakes". - Krista"I use to always use this Hershey recipe, but it’s has been years since I’ve made fudge. Tried a batch this weekend and it reminded me of how I used to fix the fails I’ve been reading about. If the fudge never sets or turns to concrete in a pan, do not despair. If too soft, heat it up again to temp (234 F for Hershey recipe – if too soft your thermometer was probably off). If it was too hard add a little milk as it melts, I cook just until melted. Remove from heat, add a pat of butter on top and let cool and beat as before. I used to call it my special twice cooked fudge recipe". Gustavo