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

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

Line an 8x8-inch baking dish with parchment paper and pour the chocolate mixture into the dish, smoothing it evenly.

Drop spoonfuls of marshmallow fluff over the chocolate layer and swirl gently with a knife or toothpick.

Refrigerate the fudge for at least 2 hours until set.

Lift the fudge out using the parchment overhang and cut into squares before serving.