In a medium saucepan, combine the semi-sweet chocolate chips, sweetened condensed milk, and unsalted butter. Place the saucepan over low heat, stirring constantly for about 5 minutes until the mixture is completely smooth and glossy.

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

Line an 8x8-inch baking dish with parchment paper and pour the warm chocolate fudge mixture into the dish.

Dollop spoonfuls of marshmallow fluff over the fudge layer and gently swirl it in with a butter knife or skewer.

Refrigerate the fudge for at least 2 hours or until it is firmly set.

Lift the fudge out of the baking dish and cut into squares. Serve chilled.