One of my all-time favorite things to eat in life, this plum cake couldn't be more sentimental. It's a classic German recipe that my grandmother made at the end of every summer during the brief window when the Italian prune plums are in season. Today, my mom and I both carry on that tradition, and I can still eat an entire tray by myself if left to my own devices. Pro-tip: Don't use anything but a 9x13 jelly roll pan for this. Anything larger and the dough won't fill the pan, any smaller and the dough will be too thick.