Italian prune plums have a short but prolific season in October. Since they are firm and not overly juicy, they take well to roasting and won't fall apart.
Preheat oven to 400 degrees. Toss plums in a large bowl with sugar and butter; place cut side down on a rimmed baking sheet. Roast until plums are cooked through and caramelized, 15 to 20 minutes. Remove from oven, and serve.