Combine all ingredients except butter and vanilla. In a 2 quart saucepan, combine the sugar, half and half, cocoa, salt, and corn syrup. Mix well with a whisk and bring to a boil over medium low heat. While the fudge is cooking, butter a plate or baking dish for pouring the mixture into lalter.
Heat until mixture reaches 240 degrees. Let the fudge cook until it reaches 240 degrees, checked with a candy thermometer or instant-read thermometer. Don't stir during the ocoking process. Once the temperature is reached, immediately remove the pan from the heat.
Place butter and vanilla on top and allow to cool. Place the butter and vanillla on top of the fudge and allow it to cool. Do not mix now or disturb the pan. Let it cool about. 20 minutes until the side of the pan is cool to the touch.
Mix with hand mixer until fudge begins to firm up. Using a handmixer on medium low, beat the fudge for 1-3 minutes until it just begins to firm up and lose its shine.
Quickly pour into buttered dish, cool, and cut. Once the. fudge changes texture, immediately pour in into a dish to set. Work quickly. It if is not pourable, just scoop it out and flatten in the best you can. Allow it to cool for 20 minutes and cut into one-inch squares.