Heat chocolate, butter, and vanilla until melted. You can use the double boiler method, or dump ingredients into a microwave-safe bowl, heating on high in 30-second increments, stirring between, until fully melted.
Add marshmallow fluff and sweetened condensed milk.
Add ¾ cup walnuts and marshmallows. Stir well.
Pour mixture onto parchment paper-lined baking pan, spread evenly.
Top with remaining walnuts.
Cover with foil and refrigerate for at least 2 hours before cutting into 1-2 inch squares.
This is a very soft fudge recipe, so you may want to freeze for one hour before cutting, for a smoother cut. You may also want to store this fudge in the refrigerator.