In a medium saucepan, combine 2 cups of semi-sweet chocolate chips, 1 can of sweetened condensed milk, and 2 tablespoons of melted unsalted butter. Place the saucepan over low heat and stir continuously for about 5-7 minutes until the mixture is completely smooth and glossy.

Once the chocolate mixture is smooth, remove the saucepan from heat and stir in 1 teaspoon of vanilla extract and a pinch of salt.

Line an 8x8-inch baking dish with parchment paper, allowing some overhang for easy removal later. Set the prepared dish aside.

Pour the warm chocolate mixture into the lined baking dish, and use a spatula to spread it evenly across the bottom.

Spoon dollops of marshmallow fluff over the top of the fudge layer, and gently swirl it through the fudge without overmixing.

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

Once the fudge has set, carefully lift it out of the dish and slice it into uniform squares.