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1
Position rack in center of oven and preheat to 400u00b0F. Butter 9- by 13-inch baking dish or 2-quart casserole. Line rimmed baking sheet with aluminum foil.
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2
Pierce each potato with fork and transfer to baking sheet. Bake until tender, about 50 minutes. Cool until easy to handle.
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3
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.
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4
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 puree, handheld electric mixer. Spread mixture in prepared dish and cool completely. (Streusel and puree can be made up to 2 days ahead and refrigerated, separately, covered.)
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5
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.)
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6
Sweet potato puree with marshmallows: Omit streusel. Spread puree 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.
