Jem Aswad Senior Music EditorIt might have a core of ultra-rich high-calorie cheese, but the throwback song is a very specific and carefully calibrated art form.It must evoke and transcend the era it’s reviving in an affectionate or hilarious fashion (or both); it has to be a great song as well as a nostalgia trip for those who remember, and a vicarious saunter through a previous decade for those who don’t.