In a microwave-safe bowl, combine sweetened condensed milk and chocolate chips. Microwave in 30-second intervals, stirring in between, until smooth and melted.
Stir in the mini marshmallows and chopped walnuts until well combined.
Pour the mixture into the prepared baking pan and spread evenly.
Refrigerate for at least 2 hours, or until firm. Cut into squares and serve.
Notes
This fudge is best stored in an airtight container in the refrigerator.