In a medium saucepan over low heat, combine semi-sweet chocolate chips, sweetened condensed milk, and unsalted butter. Stir continuously for about 5 minutes until melted and smooth.

Remove from heat, then add in vanilla extract and a pinch of salt, mixing until well incorporated.

Line an 8x8-inch baking dish with parchment paper. Pour the warm chocolate mixture into the dish and spread evenly.

Drop dollops of marshmallow fluff over the fudge layer. With a knife, gently swirl the marshmallow into the fudge for a marbled look.

Chill in the refrigerator for at least 2 hours, or until firm.

Once chilled, lift the fudge out using the parchment overhang and cut into squares.