Lightly grease an 8×8-inch baking pan. Line the pan with parchment paper or aluminum foil, allowing the excess paper to overhang the sides.

In a large, heatproof bowl combine the sweetened condensed milk, vanilla extract, chocolate, and butter; set aside.

Fill a saucepan with a little water (about 1/4 full) and bring it to a boil. Reduce the heat to barely a simmer. Place the heatproof glass bowl holding the ingredients on top of the pot.

Stir with a rubber spatula until the chocolate is almost fully melted with just a few small pieces of chocolate remaining, about 4 minutes.

Carefully remove the bowl from the heat and continue to stir until the chocolate is fully melted and the mixture is smooth, about 2 minutes.

Stir in any mix-ins if using, then scrape the fudge into the prepared pan and spread into an even layer with a spatula.

Refrigerate until set, about 2 hours. Remove the fudge from the pan using the foil overhang and cut into squares.

Gently run a butter knife around the edge of the pan to loosen the fudge. Remove the fudge and transfer it to a cutting board.

Peel off parchment paper, then slice the fudge into small pieces. Serve at once or store in the refrigerator for up to 1 month.

You may also freeze the fudge in an airtight freezer-safe container for up to two months.