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.
5. 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.

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.