Prepare the fudge base. In a 2-quart saucepan, combine the sugar, cocoa powder, half-and-half, salt, and corn syrup. Then, whisk the ingredients until smooth and well-mixed. Place the pan over medium-low heat and bring the mixture to a gentle boil, stirring occasionally.
Heat to 240°F. Allow the mixture to cook without stirring until it reaches 240°F (soft-ball stage) on a candy thermometer. This temperature is essential for setting the fudge’s texture. Once the temperature is reached, immediately remove the pan from the heat.
Add the butter and vanilla, then cool. Place the butter and vanilla on top of the hot fudge mixture, but do not stir. Let it cool undisturbed for about 20 minutes or until the side of the pan is warm but not hot to the touch. This resting time helps the fudge set up correctly.
Beat until thickened. Using a hand mixer at medium to low speed, beat the fudge mixture for 1-3 minutes or until it starts to thicken and lose its shine. This change in texture indicates the fudge is ready for pouring.
Pour and set. Quickly pour or scoop the fudge into a buttered plate or dish, using a spatula to spread it evenly if needed. Work swiftly here, as the fudge will set quickly once it cools. Then, allow the fudge to cool for about 20 minutes before cutting into one-inch squares.