- 5 pounds orange-fleshed sweet potatoes, such as Louisiana, jewel, or garnet yams
- 3/4 cup all-purpose flour
- 3/4 cup plus 1/3 cup (packed) light brown sugar
- 14 tablespoons (1 3/4 sticks) unsalted butter, softened
- Position rack in center of oven and preheat to 400°F. Butter 9- by 13-inch baking dish or 2-quart casserole. Line rimmed baking sheet with aluminum foil.
- Pierce each potato with fork and transfer to baking sheet. Bake until tender, about 50 minutes. Cool until easy to handle.
- Meanwhile, make streusel: In medium bowl, combine flour, 3/4 cup brown sugar, and 1 stick butter. Rub mixture between fingertips until blended and crumbly. Set aside.
- Halve potatoes lengthwise and scoop out flesh into large bowl. Add remaining 1/3 cup brown sugar and 3/4 stick butter and mash with potato masher or, for a smoother purée, handheld electric mixer. Spread mixture in prepared dish and cool completely. (Streusel and purée can be made up to 2 days ahead and refrigerated, separately, covered.)
- Sprinkle streusel over potatoes. Bake until golden brown, about 40 minutes. Let stand 5 minutes before serving. (Completed dish can be prepared up to 1 day ahead and refrigerated, covered. Remove from refrigerator 1 hour before reheating and let come to room temperature. Bake, loosely covered with foil, until heated through, about 25 minutes.)
- Sweet potato purée with marshmallows: Omit streusel. Spread purée in baking dish, cover with foil, and bake until heated through, about 30 minutes. Sprinkle top with 3 cups miniature marshmallows and broil until marshmallows are lightly browned, about 1 minute. Serve immediately.