Grease or spray a 9x5-inch loaf pan. Line it with parchment paper, leaving extra flaps of parchment paper on the two long sides for easy removal later.
Put the chocolate into a large, microwaveable-safe bowl. Microwave the chocolate on high at 30-second intervals, stirring in between until the chocolate is melted. It will be 2 minutes total.
Stir in the condensed milk, salt, and vanilla extract.
Pour the mixture into the prepared pan, smoothing the top.
(If you are using the optional chopped nuts, just stir the nuts into the mixture before pouring it into the pan. I like to do half plain, and half with walnuts.)
Optional: Decorate the top if you wish. Lightly press down any topping you add.
Cover the fudge and chill it in the refrigerator for 1-2 hours or until firm.
Use the extra flaps of parchment paper to transfer the fudge to a cutting board. Use a heavy, sharp knife to cut the fudge into 18 squares.
Store the fudge in an air-tight container for 1-2 weeks.