In a medium saucepan, combine 2 cups of semi-sweet chocolate chips, 1 can of sweetened condensed milk, and ¼ cup of melted unsalted butter. Heat this mixture over low heat for about 5-7 minutes, stirring continuously until everything is melted and the mixture is smooth and glossy.

Once your chocolate mixture is smooth, remove it from the heat. Stir in 1 teaspoon of vanilla extract and a pinch of salt, mixing well to ensure full incorporation. Allow the mixture to cool slightly.

Line an 8x8-inch baking dish with parchment paper. Pour your chocolate mixture evenly into the lined dish, smoothing the top with a spatula.

With the fudge mixture still warm, drop spoonfuls of marshmallow fluff across the top. Using a knife or a skewer, gently swirl through the chocolate mixture, creating beautiful ribbons of marshmallow.

Once you're satisfied with the swirling, transfer the baking dish to the refrigerator. Chill the fudge for at least 2 hours, or until it is firm and set.

After the fudge has chilled and set, lift it out of the baking dish using the parchment paper. Cut it into squares of your desired size.