Preheat the oven to 190°C (375°F) (gas 5)
Wash and halve the plums then ease out the stones.
Mix the cornflour (cornstarch) in 150ml (5 fl oz) (⅔ cup) water.
Now add to a medium-sized pan along with the demerara sugar and cinnamon. Heat gently to dissolve the sugar, stirring occasionally.
Once the sugar has dissolved, add the plums, cover the pan and simmer gently for 5–10 minutes depending on the ripeness of the plums. You want the plums to soften but still keep their shape.
Begin making the topping by sifting the flour, baking powder, and salt into a large bowl.
Now rub the butter into the flour until it resembles breadcrumbs.
Stir through the sugar.
Using a pallet knife, gradually add the buttermilk to the flour mixture to bind it together and make a slightly sticky dough. You may not need all the buttermilk.
Next tip the plums into the dish and then dollop balls of dough on top of the plums, leaving space around each one and some of the fruit exposed. The balls of dough will expand when cooking.
Bake for 25–30 minutes until the top is golden.
Let it sit for 5 minutes before serving.