In a medium saucepan, combine 2 cups of semi-sweet chocolate chips, 1 (14-ounce) can of sweetened condensed milk, and 2 tablespoons of unsalted butter over low heat. Stir continuously for about 5-7 minutes, until the mixture is completely smooth and glossy.

Once your fudge base is melted and silky, remove the saucepan from heat. Stir in 1 teaspoon of vanilla extract and a pinch of salt, mixing thoroughly to combine.

Line an 8x8-inch baking dish with parchment paper, ensuring it covers the bottom and sides for easy removal later. Pour the melted fudge mixture into the prepared dish, spreading it evenly across the bottom.

Take 1 cup of marshmallow fluff and drop spoonfuls evenly over the fudge mixture. With a butter knife or a skewer, gently swirl the fluff into the fudge, creating a marbled effect.

Cover the dish with plastic wrap or aluminum foil and refrigerate for at least 2 hours, or until the fudge is completely set.

Once the fudge is fully chilled and set, carefully lift it out of the dish using the parchment paper and cut it into 1-inch squares.