Put the dark chocolate chips, sweetened condensed milk, unsalted butter, and salt in a medium saucepan.
Cook over medium heat until melted and smooth. Stir constantly. Then turn off the heat and remove the saucepan from the burner.
Stir in the vanilla extract.
Put a sheet of parchment paper in an 8x8-inch square pan.
Pour the melted fudge mixture into the pan, then spread it evenly around the pan.
Let it sit on the countertop for about 1 to 2 hours to cool and set.
After that, remove the parchment paper from the fudge.
Cut the fudge into about 1-inch squares.
Notes
Stir the mixture constantly so it doesn't scorch on the bottom of the saucepan.
You can use a double boiler if you prefer.
Some extra add-ins to the fudge after it has melted can be coconut flakes, almonds, peanut butter, walnuts, pistachios, or pieces of dried cranberries.
If you want the fudge to be sweeter, add about 1/2 cup of semi-sweet chocolate chips to the fudge mixture.
Lightly sprinkle some sea salt over the top of the fudge after it is already in the pan and before cutting it into squares.