In a saucepan over a low fire:
2 cups sugar
2 oz. unsweetened chocolate (I just use Bakers!)
3/4 cup whole milk
2 Tablespoons white corn syrup
pinch of salt
later: 2 Tablespoons butter; 1 tsp vanilla or brandy or rum or whiskey
Simmer, try not to eat it all while it is cooking. The books say you shouldn’t stir, but I do, and then I nibble the residue that gets stuck to the side of the pan, etc. The point is the heat may do odd things to the milk, and the bottom of the pan may get sort of curdy, so don’t scrape the bottom.
You have to boil/cook it until the "soft’ or "firm" ball stage. This means you take cold water and periodically drop a bit in. If you can form a sort of firmish, squishy, not hard ball in your fingers, it is done. A "candy thermometer" might be more precise.
At that point remove from fire, let cool a bit, add 2 Tbsp ( = 1/8 cup) butter, or even a tad more, and a teaspoon of vanilla, or a bit more, or brandy. The butter melts. Meanwhile butter or grease a glass pie-plate or something like that. Let the mixture get fairly cool, i.e. you wouldn’t scald yourself if you touched it. But still warm. Then you beat it with a wooden spoon until it gets dull and starts to get thick, it may even turn solid while you are trying to pour it into the pan, but do not despair -- provided it is warm, if it is sort of thick and of the consistency you like, just get those hunks of it onto the pie plate, then get a sheet of waxed paper and an oven mitt or something and press it into place. Best, though, if soft enough to pour in and spread out nicely. If it looks like toffee, no good, too soft, although it might be nice rolled in something, or melted over icecream a/k/a "hot fudge sauce" but that, my friends, is another recipe.