In a medium saucepan, combine semi-sweet chocolate chips, sweetened condensed milk, and unsalted butter. Heat over low flame, stirring continuously for about 5 minutes, or until the mixture becomes smooth and velvety.

Once well combined and glossy, remove from heat and stir in a splash of vanilla extract and a pinch of salt.

Line an 8x8-inch baking dish with parchment paper and pour the melted fudge mixture into the dish.

Spoon generous dollops of marshmallow fluff over the fudge layer and gently swirl it in using a knife or toothpick.

Place the baking dish in the refrigerator to set for at least 2 hours.

Once set, lift the fudge from the dish and cut into squares.