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

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

Place a medium saucepan on the stove and add cold water to reach about 1/4 of the pan’s depth. Bring the water to a boil, then reduce the heat so it is barely simmering.

Set the heatproof bowl on top of the saucepan (the bottom of the bowl should not touch the simmering water). Stir the mixture continuously with a rubber spatula until the chocolate is almost fully melted and only a few small pieces remain, about 4 minutes. Remove the bowl from the heat before the chocolate is completely melted to help prevent the chocolate from separating.

Off the heat, continue stirring the mixture until the chocolate is fully melted and the mixture is smooth, about 2 minutes.

Stir in the chopped nuts or crushed canes if using. Scrape the fudge mixture into the prepared pan and spread it into an even layer with a spatula. Sprinkle flaky sea salt on top if desired.

Refrigerate the pan until the fudge is firm and set, about 2 hours.

To remove, gently run a butter knife around the edge of the pan if needed, then lift the fudge from the pan using the parchment or foil overhang. Transfer to a cutting board, peel off the parchment or foil, and cut into squares.

Serve at once or store the fudge in the refrigerator for up to 1 month. To freeze, place in an airtight, freezer-safe container for up to 2 months.