Chuzi, I'll bet this is the recipe your Grandmother used -- it is the one from the back of the original Hershey's Cocoa CAN (when it actually came in metal cans). I know, because I've held on to one of those cans for years!
2/3 cup Hershey's Cocoa
3 cups Sugar
1/2 tsp Salt
1-1/2 cups milk (whole or 2%)
1/4 cup Butter
1 tsp Vanilla extract
Directions:
Butter an 9-inch round or square pan or dish (do this even if it is non-stick).
Thoroughly combine dry ingredients in a LARGE, heavy saucepan. Stir in the
milk. Bring to a FULL ROLLING BOIL (a boil that cannot be stirred down) on medium heat, stirring constantly. Then
boil, WITHOUT stirring, until mixture reaches soft ball stage (240°F on a
candy thermometer or when a small bit dropped in cold water forms a soft ball). Bulb of candy thermometer should not rest on bottom of
pan. DO NOT STIR, but remove from heat.
Add butter and vanilla on top of mixture, but DO NOT STIR. Allow mixture to cool, at room temperature, to 110°F degrees (pan is barely warm to the touch). Now beat (a wooden spoon works best for this) until fudge thickens and begins losing its gloss. Quickly spread in pan; let harden; cut into squares.
If the fudge isn't hard enough, you didn't let it boil long enough -- if it is too hard, you let it boil too long. But don't worry, it isn't as hard as it sounds, and you can still enjoy eating your experiments! If you don't have a candy thermometer (I don't), just keep testing a little bit in cold water until you get a soft ball -- be sure to use fresh cold water for each test.
You can double this recipe, but be sure to use a BIG PAN (I use my large Dutch oven) because it will "rise" in the pan as it boils and you don't want it to boil all over your stove!
Hope this is the one you're looking for!
Judyfish