Sweet Potato Casserole with Marshmallows

Ingredients

• 6 pounds moist sweet potatoes, such as ruby or garnet yams
• 8 tablespoons unsalted butter
• 8 sprigs fresh thyme and/or sage
• 1 knob peeled fresh ginger, finely grated
• 1/4 cup buttermilk or sour cream
• Kosher salt
• 2 cups mini marshmallows

Directions

1. Move the oven rack to the center. Lay 2 large pieces of heavy-duty aluminum foil on your counter. Put half of the potatoes in the middle of one piece, then bring the foil up around them and pinch the edges closed so the packet is well sealed. Do the same with the rest of the potatoes on the second sheet. Set both packets on a rimmed baking sheet and put them in the oven. Turn the oven to 300 degrees F (149 degrees C) and roast until a slim skewer slides through the potatoes easily, about 2 hours. Take them out and let them sit until you can handle them comfortably.
2. While the potatoes roast, melt the butter in a medium saucepan over medium-low heat. Gently swirl the pan as it cooks, until the browned bits turn pale golden and the butter smells toasty. Add the herb sprigs carefully, since the butter may sputter when they go in. Off the heat, swirl the herbs in the browned butter until they crisp and frizzle. Set the pan aside.
3. Remove and discard the sweet potato skins. Put the flesh in a large bowl, or in the bowl of a stand mixer. Using the whisk attachment or a handheld mixer, whip the potatoes until they are smooth and airy, about 2 minutes. Pour the brown butter through a strainer into the potatoes, leaving the frizzled herbs behind. Beat in the ginger, then mix in the buttermilk or sour cream. Add salt to taste.
4. To finish, heat the oven to 375 degrees F (190 degrees C). Spoon the sweet potatoes into a 2-quart baking dish and even out the top with a rubber spatula. Bake until hot all the way through, about 30 minutes. Raise the oven temperature to 475 degrees F (246 degrees C), scatter the mini marshmallows over the top, and bake until they melt and turn lightly golden, about 3 minutes. Watch closely here, because they can go from browned to burned fast. Serve.