Preheat the oven to 350°F (175°C). In a mixing bowl, combine sliced plums, granulated sugar, cornstarch, and vanilla extract. Toss until plums are evenly coated.

In another bowl, mix rolled oats, flour, brown sugar, melted butter, and cinnamon until crumbly.

Spread the plum mixture in a baking dish and evenly sprinkle the oat topping over the plums. Bake for 30 minutes or until the topping is golden brown and the plums are bubbly.

Let cool slightly before serving. Enjoy warm, optionally with vanilla ice cream.