In a large bowl, cut the chocolate chips into the butter and set aside. Bring the sugar, milk and 20 marshmallows to a boil over medium heat. Boil for five minutes, stirring constantly. Pour this mixture over the butter and chocolate chips. Stir until thick. And nuts, vanilla, and frozen marshmallows. Pour into buttered 9 by 13-inch pan. Chill until firm. Cut into squares when chilled.