Place prunes and water in a large saucepan and let rest in refrigerator 8 hours or overnight.
Cook prunes in same saucepan with soaking water over medium heat, covered, 1 hour, until soft. Let cool.
In a small saucepan over medium heat, cook apples until soft, about 20 minutes.
Reserve prune water and remove pits from prunes. Puree pitted prunes with cooking water in a food processor or blender until smooth. Combine prune puree and cooked apple in a large saucepan with the cinnamon stick. Combine the potato flour with the half-and-half and stir into fruit mixture. Bring to a boil, then stir in sugar. Reduce heat to medium-low, stir in grape juice and raisins, and partially cover. Let simmer until thoroughly heated, then remove from heat.