1. Preheat oven to 400°F. Toss sweet potatoes and oil together until evenly coated on a large rimmed baking sheet. Cover tightly with foil. Roast until sweet potatoes are very tender, about 45 minutes.
2. Meanwhile, bring raspberries, brown sugar and water to a simmer in a nonreactive medium saucepan over medium heat. Cook, stirring often, until the raspberries are soft and their juices are slightly thickened, about 10 minutes. Purée in a blender. Strain through a fine mesh sieve into a bowl. You should have 3/4 cup raspberry purée; simmer over medium heat to reduce, if needed.
3. Transfer sweet potatoes to a large bowl. Add butter, maple syrup and 1/2 cup raspberry purée; reserve remaining purée. Whip with an electric mixer on high speed until smooth. Season lightly with salt.
4. To serve, transfer to a warm serving bowl. Drizzle with the remaining raspberry puree. Serve hot.

If Making Ahead

1. The whipped sweet potatoes and raspberry purée can be cooled, covered, and refrigerated for up to 1 day. Let raspberry puree stand at room temperature for 1 hour. Reheat sweet potatoes gently in a large saucepan over low heat, stirring often.
2. Note: Grade B maple syrup has stronger maple flavor than Grade A syrup. It is available at natural food stores and some wholesale price clubs.