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, set aside.

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

Fill a saucepan with 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, about 4 minutes. Remove the fudge from the double boiler before the chocolate is fully melted.

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. Freeze the fudge in an airtight freezer-safe container for up to two months.