Sweet Tatar Casserole

Directions

1. Adjust oven rack to center position. Place 2 large sheets of heavy-duty aluminum foil on a work surface. Working with one sheet at a time, place half of potatoes in center. Fold up foil and crimp edges to seal tightly. Repeat on second sheet with rest of potatoes. Transfer pouches to a rimmed baking sheet and place in oven. Set oven to 300°F (149°C). Roast until a thin skewer inserted into potatoes goes in with no resistance, about 2 hours. Remove from oven and set aside until cool enough to handle.
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3. Meanwhile, melt butter in a medium saucepan over medium-low heat, swirling gently until particles are pale golden brown and smell nutty. Add herb sprigs; herbs may cause brown butter to splatter at first, so take care. Swirl herbs in brown butter off-heat until frizzled. Set aside.
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5. Peel sweet potatoes and discard skins. Add flesh to a large bowl or the bowl of a stand mixer. Beat with the whisk attachment or with a handheld mixer until smooth and fluffy, about 2 minutes. Strain brown butter into potatoes, discarding frizzled herbs. Beat in ginger, then beat in buttermilk or sour cream. Season to taste with salt.
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7. When ready to finish dish, preheat oven to 375°F . Scrape potatoes into a 2-quart baking dish, smoothing surface with a rubber spatula. Bake until heated through, about 30 minutes. Increase oven temperature to 475°F (246°C) and top with mini marshmallows. Bake until marshmallows are melted and lightly browned, about 3 minutes (keep an eye on them, as they can brown and then burn very quickly).