Line an 8×8 or 9×9 inch baking pan with parchment paper and grease it lightly.

In a medium saucepan over low heat, combine chocolate chips, sweetened condensed milk, and butter. Stir constantly for 5-7 minutes until smooth and glossy.

Remove from heat and stir in pure vanilla extract and salt (if using). Ensure everything is combined thoroughly.

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

Dollop marshmallow crème over the fudge base and space out the dollops.

Gently swirl the marshmallow crème into the chocolate with a toothpick or knife.

Chill the fudge in the refrigerator for at least 2 hours.

Lift the fudge out using the parchment paper, slice into squares, and serve.