It's a reference to Proust's Remembrance of Things Past, wherein he famously recounted how, upon eating a "dispiriting" madeline as an adult, he was abruptly an

It's a reference to Proust's Remembrance of Things Past, wherein he famously recounted how, upon eating a "dispiriting" madeline as an adult, he was abruptly and involuntarily taken back to the emotions of childhood, when he and an aunt shared madelines every Sunday. Since then, it's shorthand for "something that emotionally transports us back to happier times," more or less.