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

Once your fudge base is smooth, remove the saucepan from the heat. Stir in 1 teaspoon of vanilla extract and a pinch of salt.

Line an 8x8-inch baking dish with parchment paper, leaving some overhang. Pour the melted chocolate mixture into the prepared dish, spreading it evenly.

Spoon dollops of marshmallow fluff over the fudge mixture. Using a knife or toothpick, gently swirl the marshmallow into the chocolate.

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

Once chilled, lift the fudge out of the dish using the parchment paper overhang. Transfer it to a cutting board and slice it into squares.