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

Add in the vanilla extract and a pinch of salt to the melted fudge mixture. Mix until fully combined and fragrant, ensuring not to overmix.

Line an 8x8-inch baking dish with parchment paper and pour the fudge mixture into it, spreading evenly. Let it cool slightly.

Dollop spoonfuls of marshmallow fluff over the fudge layer and gently swirl with a butter knife to create beautiful ribbons.

Cover the dish with plastic wrap and refrigerate for at least 2 hours, until firm to the touch.

Once set, use the parchment paper to lift out the fudge from the dish, place on a cutting board, and slice into squares.