Line a 9x5 loaf pan with parchment paper and set aside.

Lightly dampen a pastry brush and set it nearby the stove.

In a medium-sized, heavy-bottomed saucepan, combine sugar, milk, corn syrup and chocolate over medium heat.

Using a wooden spoon, stir the mixture constantly, occasionally brushing the side of the pot with the pastry brush to remove crystals that might fall into your fudge.

Once mixture begins to boil, attach your candy thermometer to the pot (make sure the bottom of the thermometer isn't touching the bottom of the pan).

Stir mixture occasionally until your thermometer reads 238F (soft ball stage).

Immediately remove pan from heat and add your butter and vanilla extract. Do not stir the butter and vanilla! Allow it to set and melt

Leave mixture undisturbed until your candy thermometer reads 110F. This will take a long time, possibly an hour or more.

Now, using a (clean) wooden spoon, begin to stir the mixture vigorously (be careful not to scrape the sides or you may knock sugar crystals into the fudge, causing grainy fudge) until it begins to lose its shiny sheen and thicken (It will take a good bit of stirring before the fudge is just right, your arms will get tired and it's not a bad idea to have a back-up stirrer... seriously!)

Immediately, once the fudge begins to thicken, pour into prepared pan.

Allow to cool completely before cutting and serving.