In a saucepan over low heat, melt the semi-sweet chocolate chips, sweetened condensed milk, and unsalted butter. Stir constantly until the mixture is smooth and creamy, about 5-7 minutes.

Remove the pan from the heat and stir in the vanilla extract and salt. Mix until everything is fully incorporated.

Line an 8×8-inch baking dish with parchment paper, making sure it hangs over the edges for easy removal later. Spread the chocolate fudge mixture evenly into the pan.

Dollop spoonfuls of your marshmallow fluff or cream over the top of the fudge. Use a knife to gently swirl it into the chocolate, creating a beautiful marbled effect.

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

Once chilled and set, lift the fudge out using the parchment paper. Cut it into neat squares with a sharp knife and enjoy.