Combine butter, chocolate pieces and vanilla in a medium bowl. Set aside. Place sugar evaporated milk and marshmallows in medium saucepan. Bring to a boil over medium heat use a candy thermometer to make sure you reach the soft ball stage, stirring frequently.
Reduce heat to low and cook 6 minutes stirring constantly. Pour hot mixture over in bowl. Beat with electric mixer until fudge is thick and dull (this doesn't take long). Stir in nuts. Pour into lightly buttered 8-inch square baking pan. Refrigerate several hours to firm.