Fill a large stainless steel saucepan with sugar, chocolate, heavy cream, and light corn syrup.
Mix together with a large stainless steel spoon over medium-low heat until the chocolate has all melted and it starts to bubble.
Continue to stir the chocolate mixture until it doubles in size and starts to shrink back down again as it thickens.
When the chocolate mixture starts to shrink down and coat the spoon, it's time to test if it's reached the soft-ball stage. Prepare a small bowl filled half-way with water. (You can skip this if you plan to use a candy thermometer...but this is a fun way to test your fudge.)
Stir the chocolate mixture and let a few drops of chocolate drip into your bowl of water. If the chocolate spreads out on the bottom of the bowl, it's not ready yet. Pour the water out (taste the chocolate if you want) and fill the bowl with water again.
The soft-ball stage is reached in a matter of seconds once the chocolate shrinks and coats your spoon, so every 30 seconds drop a few drops of chocolate in your bowl of water. You will start to notice the chocolate sticking together more. Do this until you reach the soft-ball stage.
The soft-ball stage has been reached when your chocolate drips turn into balls in your bowl of water. Feel the chocolate with your finger in the water and push it around. It should feel soft and not hard like caramel.
Remove the saucepan from the heat when it reaches the soft-ball stage.
Add butter to the top but don't stir it in. Set your timer for 10 minutes.
When your timer goes off, add in your vanilla and start stirring the chocolate fudge. Stir about 5-10 minutes until the shine starts to go away and when the fudge starts to hold a shape in your saucepan.
Get ready to pour out your fudge by placing some wax paper down on a cutting board or cookie sheet.
Pour your chocolate fudge out onto your wax paper and smooth the top with your spoon the best you can. Let it sit undisturbed until it cools down (about 20-30 minutes).
Cut your fudge into squares using a sharp knife. Place another sheet of wax paper on top of your fudge and then grab the cutting board or cookie sheet and flip it all over upside down onto your counter. Slowly remove the wax paper from the top of your fudge so that the underside (now the top) can also cool down and harden.
Store fudge squares in an air-tight container. It's best served warm or at room temperature and eaten within 3 days, but you can also store it in your refrigerator to make it last longer.