Old-Fashioned Cooked Fudge

Combine cocoa with sugar in heavy saucepan and mix with wire whisk. Add milk and corn syrup and mix well. Bring to boil over medium heat, stirring frequently. When mixture comes to a boil, cover pan for 2 minutes so sugar crystals are washed down sides of pan by the steam. Uncover pan, clip on a candy thermometer, making sure it doesn't touch the bottom of the pan, reduce heat to low and cook gently to 232 degrees (soft ball stage). Remove from heat, drop butter into the mixture, and let the saucepan sit without moving until lukewarm (110 degrees). This is a very important step - if you move or jostle the pan while the fudge is cooling you'll have sugary fudge. Then add vanilla and beat with spoon until fudge begins to lose its gloss and thickens. This takes awhile, and you'll probably need help stirring! All at once it will come together and look like fudge. The candy will not dissolve back into the mixture in the pan when you lift the spoon but will leave a distinct ribbon of fudge. Quickly spoon and spread into a 9" greased square pan. Let cool completely and cut into squares.