In a medium saucepan over low heat, combine semi-sweet chocolate chips, sweetened condensed milk, and unsalted butter. Stir constantly for about 5–7 minutes, until the mixture is completely smooth and glossy.

Once the fudge base is smooth, remove the saucepan from heat. Stir in vanilla extract and salt, mixing well for about 30 seconds.

Prepare an 8x8-inch baking dish by lining it with parchment paper.

Pour the melted chocolate mixture into the prepared baking dish and spread it evenly.

Dollop spoonfuls of marshmallow fluff over the surface and gently swirl it into the chocolate mixture.

Cover the baking dish with plastic wrap and chill for at least 2 hours until firm.

Once chilled, lift the fudge out of the dish and slice it into squares.