Line an 8x8 inch baking pan with parchment paper, leaving an overhang on the sides.

In a medium saucepan over low heat, combine chocolate chips, sweetened condensed milk, and salt.

Stir continuously until the chocolate is completely melted and the mixture is smooth.

Remove from heat and stir in the vanilla extract.

Spread the chocolate mixture evenly into the prepared baking pan.

Drop small dollops of marshmallow fluff onto the top of the fudge.

Using a butter knife or toothpick, swirl the marshmallow fluff into the chocolate layer.

Refrigerate for at least 2 hours or until firm.

Lift the fudge out of the pan using the parchment paper and cut into squares.