Grab a large bowl and cut the chocolate chips into the butter. Set aside. Now combine the sugar, milk and the 20 marshmallows to a boil over medium heat. Boil for 5 minutes, stirring constantly. Pour this mixture over the chocolate chips and butter mixture. Stir until thick. Add the nuts and the vanilla and the frozen marshmallows. Pour into buttered 9-inch by 13-inch pan. Chill until firm. Cut into squares.