The file
A plain shortbread cookie that turned out to have fudge inside. The Elves sold it as magic, and to a seven-year-old it was. Magic Middles are the most-requested item in every "bring back Keebler" thread.
The usual explanation is that the filling required a special line, and when Keebler needed the capacity the cookie went. There's been no revival, only knockoff recipes and a lot of people wistfully describing the center.
Sources: Wikipedia — Keebler Company
