Have you ever wished it possible to block someone in real life, just like you can on Twitter? You're not the only one. Charlie Booker's notoriously chilling and unsettling Black Mirror returns for a special holiday episode “White Christmas” that explores that exact concept. “White Christmas” will star Jon Hamm (Mad Men) and Oona Chaplin (Game of Thrones) and will air on the U.K.'s Channel 4 on Christmas Day.