When you make fudge, first line an 8-inch square baking pan with aluminum foil and coat with butter or cooking spray. Set the pan aside.
Combine sugar, cocoa, salt, milk, and corn syrup in a two-quart saucepan. Bring to a boil over medium-high heat, stirring gently to help dissolve the crystals, but being careful not to slosh them all over the sides of the pan. Wash down any crystals clinging to side of pan with a brush dipped in warm water. After you wash the crystals away, insert the candy thermometer.
Cook until the thermometer registers 234 degrees. This is the soft ball stage.
Remove from heat; add butter and let the butter melt without stirring it.
Let the mixture cool for 10 minutes then stir in vanilla. Transfer the mixture to a glass bowl. Don’t scrape the sides of the pan when you’re pouring the fudge into the bowl, it may become grainy.
Beat with a mixer at medium-low speed for two to three minutes or until the mixture just begins to lose its gloss and starts to feel thicker. Working quickly, pour the fudge into the prepared pan and let it cool completely.
When the fudge is cool and firm, lift the foil out of the pan and place it on a cutting board. Fold back the edges of the foil, and cut into 1-inch pieces.