Lightly grease an 8×8-inch baking pan. Line with parchment paper or aluminum foil, leaving an overhang.

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

Fill a saucepan with about 1/4 full of water and bring to a boil. Reduce heat to a simmer; place the bowl over the pot.

Stir with a rubber spatula until chocolate is nearly melted, about 4 minutes.

Carefully remove the bowl and stir until smooth, about 2 minutes.

Stir in mix-ins if using, then spread fudge evenly in prepared pan.

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

If desired, use a butter knife to loosen the edges before transferring to a cutting board.

Peel off parchment paper and slice into pieces. Serve immediately or store.

If freezing, use an airtight container for up to two months.