In a medium saucepan, combine semi-sweet chocolate chips, sweetened condensed milk, and melted unsalted butter. Heat this mixture over low heat, stirring constantly for about 5–7 minutes or until everything is melted and smooth.

Once the fudge mixture is smooth, remove the saucepan from heat and stir in vanilla extract and a pinch of salt. Mix thoroughly for about 1–2 minutes until well incorporated.

Line an 8x8-inch baking dish with parchment paper, allowing some overhang for easy removal later. Pour the melted chocolate mixture into the lined dish and spread it evenly.

Dollop spoonfuls of marshmallow fluff evenly across the top of the fudge mixture. Using a knife, gently swirl the fluff into the chocolate.

Place the baking dish in the refrigerator and let the fudge chill for at least 2 hours.

Once set, lift the fudge out of the baking dish using the parchment paper. Place it on a cutting board and slice it into even squares.