Place butter, chocolate chips and vanilla in a mixing bowl. Set aside.
In a medium saucepan over medium heat, combine ,milk, sugar, and marshmallows. Bring to a boil, stirring constantly. Candy thermometer temperature: 225-227 degrees. Reduce heat to low and cook 6 minutes more, stirring constantly without scraping sides and/or bottom of pan. Remove from heat.
Pour marshmallow mixture over contents of mixing bowl. Do not scrape out pan. Beat entire mixture until it thickens and loses its gloss. Quickly fold in nuts and pour into prepared pan. Refrigerate several hours until firm.