How to Make It
In a large nonreactive saucepan, combine the potatoes, cider, brown sugar, 6 tablespoons of the butter and the cinnamon stick. Bring to a boil over moderate heat. Reduce the heat to a simmer, partially cover and cook, stirring occasionally, until the potatoes are very tender, about 45 minutes.
Let cool slightly, remove the cinnamon stick and pass the potatoes through the medium disk of a food mill or puree in batches in a food processor. Transfer to an ovenproof serving dish.
Preheat the oven to 325°. Dot the potatoes with the remaining 2 tablespoons butter, cover with foil and bake, stirring once or twice, for about 20 minutes, until steaming. Remove the foil and bake for 5 minutes longer.