6 pounds sweet potatoes, peeled and cut into 2-inch chunks
6 cups apple cider
4 cups water
1 stick (4 ounces) unsalted butter
2/3 cup pure maple syrup
In a large saucepan, combine the sweet potatoes with the cider and water and bring to a boil. Simmer over moderate heat until the sweet potatoes are just tender, about 35 minutes. Drain in a colander and shake dry.
In the same large saucepan, bring the butter and maple syrup to a boil. Add the potatoes and toss. Working in batches, puree the sweet potatoes in a food processor; do not overprocess. Season with salt. Return the whipped sweet potatoes to the saucepan and heat over moderate heat until warmed through, about 10 minutes.
The whipped potatoes can be refrigerated overnight and reheated in a buttered baking dish in a 350° oven for 25 minutes or over moderate heat on the stovetop for 10 minutes.