Preheat the oven to 200C. Prick the potatoes and bake in their skins for 30-45 minutes until cooked. Leave to cool then peel and mash thoroughly.
Add the egg to the potato mash along with the flour to make a dough. Leave to rest in the fridge for an hour.
Roll out the dough and cut rounds with a 7cm pastry cutter – you will need two rounds for each plum half. Put one plum half cut side facing up on a pastry round then fill the hole with some of the cinnamon and Demerara sugar. Place another circle on top and press both circles together so that the plum is sealed in the pastry.
Bring a large saucepan of salted water to the boil and lower the dumplings into it carefully with a slotted spoon. Simmer for 5 minutes then remove the dumplings and drain.
Heat the vegetable oil in a deep frying pan and fry off the breadcrumbs and 50g Demerara sugar until brown and crisp. Finally, add the dumplings one at a time and roll them around the crumb mixture until coated. Drizzle with a chocolate sauce and serve immediately with ice cream or custard.