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.