Butter an 8-inch square baking dish, and set it aside.

Combine all the chocolate and the half-and-half in a medium saucepan, and warm it over low heat, stirring until the chocolate is completely melted. Add the sugar, corn syrup, and salt, and cook to the soft-ball stage (236° to 240°F), 15 to 20 minutes (see note). Remove the pan from the heat.

Add the butter and do not stir. Allow the fudge to sit, undisturbed, until it reaches 120°F. Then add the vanilla and the pecans, if using, and beat with a hand-held electric mixer until the fudge loses its gloss and starts to thicken but is still warm enough to pour into the prepared baking dish. The fudge will look less like chocolate sauce and more like chocolate frosting. Spread it out evenly in the dish, and let it cool completely. Then cut it into 1-inch cubes.

Note: Use your thermometer, but another way to gauge the soft-ball stage of the candy is by using an ice bath: When you are ready to test if the fudge has cooked long enough, remove it from the heat and drop a small dollop into the ice bath. After it has cooled for a few seconds, it should form a soft, malleable mass when gripped between your fingertips. This is called the soft-ball stage, and this is what you want for the perfect fudge.