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.
When I was younger, I adored candy. I'd chew on anything gummy, lick my way through a Gobstopper's Technicolored layers, and trade in all my credits for Charleston Chews at my summer camp's canteen. I was, to put it simply, a totally normal sugar-obsessed kid, albeit one who never got a single cavity—an achievement which I continue to be childishly proud of, even at 37.
But just like diapers, G.I. Joe, and brutal fistfights with my sister, my addiction to sucrose was something I eventually outgrew. That presents a problem with a dish like classic marshmallow-topped sweet potato casserole, with its layers of achingly sweet atop achingly sweet. I know a lot of people, in fact, who refuse to eat it for exactly this reason. But I'm not quite so ready to give up on it.
The biggest problem with most recipes is that they instruct you to sweeten the sweet potatoes as much as you would if you were eating them alone. A hefty dose of brown sugar or maple syrup is a common addition. That's a mistake. Remember: We're going to cover these sweet potatoes with a blanket of intense sugariness, so why in the world would we make the potatoes themselves candy-sweet as well?
That doesn't mean the sweet potatoes shouldn't be sweet, though. Just as when you pair a sweet wine with dessert, it's important that there isn't too much of a mismatch between the two parts—the juxtaposition makes extreme differences that much more glaring.
So what to do? The first step is to take advantage of the sweet potatoes' natural sweetness by cooking them right. If you think back to Kenji's mashed sweet potato recipe, you'll remember that the key is to cook the potatoes at a low enough heat that their natural enzymes break down their complex starches into simple sugars. Cook 'em too hot and you'll shut down that enzymatic activity, ending up with potatoes that taste far less sweet than they otherwise would. By roasting the sweet potatoes in a relatively low oven, set to 300°F (149°C), we'll get sweeter results than if we crank the dial higher.
If you have an immersion circulator, you can maximize this enzymatic process by first cooking the potatoes sous vide in 150°F (66°C) water for two to four hours, then proceeding with the roasting as directed.
Once the potatoes are done, simply scoop the flesh out of the skins into a bowl and beat them with an electric mixer.
The next step is to decide what to do with those extra-sweet mashed potatoes. My goal is to play up some more savory notes to add complexity instead of just, you know, more confectionery.
First up, I mix in brown butter that I've infused with herbs like sage or thyme. The brown butter adds nutty notes, along with plenty of richness, while the frizzled herbs add a woodsy aroma.
Next, I grate in fresh ginger. It adds a spicy, peppery background heat that helps offset the marshmallow sweetness on top. Minced candied ginger would be a good play here, too, if you want to go that way.
Then, for a final touch, I add a small amount of buttermilk or sour cream. As exemplified by lemonade, Daiquiris, and Sour Patch Kids, one of the best ways to make something sweet palatable is to balance it with something tart and vice versa. The subtle lactic tang of these cultured dairy products does exactly that here. Now, mind you, I don't want the potatoes to be tangy; I just want a hint of brightness to stand up against that sugary topping.
I prefer to use mini marshmallows for the topping, since they create a more elegant layer on top of the sweet potatoes. But here's the best part: Because we've carefully modulated the sweetness of the potatoes and balanced them with rich, nutty, and savory flavors, we don't have to go light on the 'mallows. So load 'em on. That kind of childlike abandon is half the fun of this dish, after all.
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
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).
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.
|Nutrition Facts (per serving)|
|Servings: 6 to 8|
|Amount per serving|
|% Daily Value*|
|Total Fat 14g||17%|
|Saturated Fat 8g||41%|
|Total Carbohydrate 80g||29%|
|Dietary Fiber 11g||40%|
|Total Sugars 29g|
|Vitamin C 67mg||334%|
|*The % Daily Value (DV) tells you how much a nutrient in a food serving contributes to a daily diet. 2,000 calories a day is used for general nutrition advice.|