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

Remove from heat and stir in vanilla extract and salt. Whisk until well combined.

Line an 8x8-inch baking dish with parchment paper. Pour melted fudge into the dish and spread evenly.

Spoon dollops of marshmallow fluff over the fudge. Use a butter knife to swirl gently.

Refrigerate for at least 2 hours or until firm to the touch.

Lift out of the dish using parchment overhang. Cut into squares to serve.