Lightly grease an 8×8-inch baking pan, then line it with parchment paper or aluminum foil, allowing the excess paper to overhang the sides for easy removal later. Set aside.

In a large, heatproof bowl, combine sweetened condensed milk, vanilla extract, chopped semi-sweet chocolate, and unsalted butter. Set the bowl aside.

Fill a saucepan about 1/4 full with water and bring it to a boil. Once boiling, reduce the heat to a barely simmering level.

Place the heatproof bowl containing the fudge ingredients over the saucepan, ensuring the bottom of the bowl does not touch the water. Stir continuously with a rubber spatula until the chocolate is almost fully melted, leaving just a few small pieces, about 4 minutes.

Remove the bowl from the heat and continue stirring until the chocolate is completely melted and the mixture is smooth, approximately 2 more minutes.

Stir in any optional mix-ins (chopped nuts or crushed candies) if using. Then scrape the fudge mixture into the prepared baking pan and spread evenly with a spatula.

Refrigerate the fudge until completely set, about 2 hours.

Once set, gently run a butter knife around the edges of the pan to loosen the fudge. Lift the fudge out using the overhanging parchment or foil, peel off the lining, and transfer to a cutting board. Slice into squares and serve.

Store leftovers in the refrigerator for up to 1 month or freeze in an airtight container for up to 2 months.