Why It Works
- Slow-roasting the sweet potatoes activates endogenous enzymes that bring out the potatoes' natural sweetness.
- Brown butter, ginger, woodsy herbs, and tangy dairy add savory notes to balance the intensely sweet marshmallow topping.
The problem with most marshmallow-topped sweet potato casseroles is that they're far too sweet, with candied sweet potatoes layered high with sugary melted marshmallows. This version corrects that by adding savory notes to the sweet potatoes, like peppery grated ginger, nutty brown butter, woodsy herbs, and just a touch of tangy dairy. That means you can go heavy on the marshmallows without your teeth quitting in protest.
How to Scale Down This Recipe to Feed a Smaller Crowd
This recipe can be scaled down by half. To do it: Divide all ingredients by two, except for the marshmallows, which should be kept at the 2 cups listed; use 1 large sheet of aluminum foil instead of 2. Bake the casserole in an 8-inch skillet.
- 6 pounds (2.8kg) moist sweet potatoes, such as ruby or garnet yams (about 6 large potatoes; see note)
- 8 tablespoons unsalted butter (1 stick; 115g)
- 8 sprigs fresh thyme and/or sage
- 1 (4-inch) knob peeled fresh ginger, finely grated (about 1 tablespoon; 15g)
- 1/4 cup (60g) buttermilk or sour cream
- Kosher salt
- 2 cups mini marshmallows (3 ounces; 90g) (see note)
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.
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.
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.
When ready to finish dish, preheat oven to 375°F (190°C). 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). Serve.
Sweet potatoes can be made even sweeter if you pre-treat them in a water bath to activate their enzymes. To do this with a sous vide–style circulator, set your circulator for 150°F (66°C). Place the sweet potatoes in the water bath and let circulate for 2 to 4 hours before proceeding with the recipe. To do it with a beer cooler, fill your cooler with water at 170 to 175°F (77 to 79°C). Add the potatoes, cover, and let rest for 2 to 4 hours before proceeding. Most marshmallows are not vegetarian, since they contain gelatin; if you want to ensure this recipe is vegetarian, make sure to buy vegetarian marshmallows.
To Make Ahead: Mashed sweet potatoes can be refrigerated for up to 5 days. To store and reheat, transfer sweet potatoes to a heavy-duty zipper-lock bag with the air pressed out. Reheat by completely submerging zipper-lock bag in a pot full of hot (not boiling) water, removing the bag occasionally to squeeze contents around, until fully reheated, about 30 minutes. Or, reheat with a sous vide–style circulator set at 150°F (66°C).