My old Hershey's Cocoa can has this recipe which is the same one my Mom always used.

RICH COCOA FUDGE

3 cups sugar
2/3 cup Hershey's unsweetened Cocoa powder
1/8 tsp. salt
1 1/2 cups milk
1/4 cup (1/2 stick) butter
1 tsp. vanilla extract

Butter 8 or 9-inch square pan, set aside.

In heavy 4 qt. saucepan, stir together sugar, cocoa powder and salt; stir in milk. Cook over medium heat, stirring constantly, until mixture comes to a full rolling boil. Boil, without stirring, until mixture reaches 234 degrees on candy thermometer or until small amount of mixture dropped into very cold water, forms a soft ball which flattens when removed from water. (Don't rest bulb of thermometer on bottom of saucepan!)

Remove from heat. Add butter and vanilla. DO NOT STIR. Cool at room temperature until lukewarm (110 degrees).

Beat with wooden spoon until fudge thickens and just begins to lose some of its gloss. Quickly spread into prepared pan; cool completely and cut into squares.