Preheat the oven to 400F (200C conventional) or 350F (180C fan). Lightly grease a 9x13-inch baking dish with softened butter, paying particular attention to the sides.

Halve the plums, remove the stones, and cut the fruit into wedges. There is no need to peel them.

Add the plum wedges to the prepared baking dish. Add the granulated sugar, cornstarch, lemon juice, vanilla, cinnamon (if using), and salt. Toss until the plums are evenly coated, then spread them into an even layer.

In a large bowl, mix the flour, rolled oats, brown sugar, cinnamon, and salt until combined.

Add the cold cubed butter to the dry ingredients. Using your fingertips or a pastry cutter, rub or cut the butter into the mixture until it forms coarse crumbs with some larger buttery clumps remaining. Do not work it into a smooth dough.

Scatter the crisp topping evenly over the plums. Do not press or pack it down.

Bake for 35 to 45 minutes, until the topping is golden brown and the plum filling is bubbling around the edges. If the topping browns too quickly before the fruit is bubbling, loosely cover the dish with aluminum foil.

Allow the plum crisp to rest for at least 15 to 20 minutes after baking. The filling will thicken slightly as it cools. Serve warm with vanilla ice cream or custard.