Line an 8x8 or 9x9 square pan with parchment paper and coat with cooking spray.
In a microwave safe bowl, combine the sweetened condensed milk and chocolate chips. Microwave on high for 5 minutes.
Remove and whisk vigorously, adding the vanilla. If there are still small chunks, return to microwave for 30 seconds.
Fold in most of the walnuts, reserving about 2 tablespoons, and spread into the prepred pan while still hot. Top with the marshmallows and remaining walnuts, press down lightly into fudge mixture.
Place in the refrigerator to set for an hour. Cut into 16 small squares.
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