In a medium saucepan, combine chocolate chips, sweetened condensed milk, and butter over low heat. Stir continuously for about 5-7 minutes until melted and smooth.

Remove from heat, mix in vanilla extract and a pinch of salt, and stir for an additional minute until combined.

Line an 8x8-inch baking dish with parchment paper, leaving overhang for easy removal.

Pour the melted chocolate mixture into the prepared baking dish and spread evenly. Let it cool at room temperature for about 10 minutes.

Drop spoonfuls of marshmallow fluff over the chocolate layer and swirl gently with a knife to create ribbons.

Cover with plastic wrap and refrigerate for at least 2 hours until set.

Lift out the fudge using the parchment paper, slice into squares, and serve.