Preheat the oven to 130C / 300F / Gas 2.
Pour the prune juice into a saucepan. Add the cinnamon, ginger and vanilla pod and bring to a boil.
Meanwhile, peel the pears, leaving the stalk in place. Nestle them snugly in a small baking dish. Ensure that they are in one layer so that they cook evenly.
Pour over the prune juice and spices, cover with a lid and pop into the oven for 3 hours, turning once halfway through the cooking time.
When cooked, remove the pears into a serving dish and keep warm.
Pour the juice back into the pan, removing the spices.
Cut the vanilla pod down the length and scrape out the seeds with a small knife. Add the seeds to a large mixing bowl and pour over the whipping cream. Whip until the cream thickens and forms peaks that stand up by themselves.
Mix the arrowroot in a cup with a little cold water until it is fully dissolved.
Pour into the pan with the juice and bring to a gentle boil, stirring constantly to prevent it burning.
Within a couple of minutes the juice will thicken into a silky, rich sauce.
Pour the spiced prune sauce over the pears, and serve hot or cold with the vanilla cream.