In a medium saucepan over low heat, combine semi-sweet chocolate chips, sweetened condensed milk, and unsalted butter. Stir constantly for about 5-7 minutes, ensuring the mixture becomes smooth and glossy without burning.

After taking the saucepan off the heat, add in the vanilla extract and a pinch of salt. Stir the mixture gently for another minute until the flavors meld together beautifully.

Line an 8x8-inch baking dish with parchment paper, leaving some overhang for easy removal later. Pour the chocolate mixture into the prepared dish, using a spatula to spread it evenly across the bottom.

Spoon dollops of marshmallow fluff all over the chocolate base and gently swirl it through the chocolate, creating beautiful ribbons.

Place the baking dish in the refrigerator and allow the fudge to chill for at least 2 hours, or until fully set and firm to the touch.

Once the fudge has set, carefully lift it out of the baking dish using the parchment paper edges and slice it into squares.