By An Edible Mosaic
Add the chocolate chips, sweetened condensed milk, and butter to a large microwave-safe bowl and microwave in 30-second intervals to melt the chocolate. Stir together to combine.
Stir in the vanilla and salt.
Fold in the walnuts and marshmallows.
Transfer the fudge to the prepared pan. Spread it out evenly. Refrigerate until fully chilled, about 4 hours, before slicing and serving.