In a medium saucepan set over low heat, combine the semi-sweet chocolate chips, sweetened condensed milk, and unsalted butter. Stir continuously for about 5-7 minutes until smooth and glossy.

Remove the saucepan from heat and stir in the vanilla extract and a pinch of salt until well incorporated.

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

Spoon dollops of marshmallow fluff over the fudge layer and gently swirl through to create a marbled effect.

Cover the baking dish and refrigerate for at least 2 hours until the fudge firms up.

Lift the fudge out using the parchment paper overhang, slice into 1-inch squares, and serve.