Line an 8-inch square baking pan with parchment paper or aluminum foil, leaving an overhang on the sides for easy removal.
In a heavy-bottomed saucepan, combine the sweetened condensed milk, chocolate chips, butter, and salt over low heat. Stir constantly until everything is melted and smooth.
Remove the pan from the heat and stir in the vanilla extract.
Pour the mixture into the prepared pan and spread it evenly with a spatula. Sprinkle chopped nuts or other toppings on top if desired.
Chill the fudge in the refrigerator for at least 2 hours, or until it’s firm.
Once the fudge is set, lift it out of the pan using the overhang on the sides, and cut it into small squares.