Sweet Potato Cake with Marshmallow Frosting

I’m not sure whether this qualifies as a seasonal (fall) dessert. Sweet potatoes are available year-round, right? And so are spices? So I made this in January, and I’d eat it pretty much any time of the year, though I concede it wouldn’t be the best thing on a hot summer day. (I tried an apple marzipan cake that similarly *could* have been seasonal even though there are apples year-round, but it stuck to the pan a bit and came out too dry even though I was watching it pretty closely in the oven, so never mind that one). The original version of this sweet potato cake, from Smitten Kitchen, has a beautiful marshmallow frosting that is piped on top in dollops, then toasted a bit with a blowtorch. I don’t have a blowtorch (nor do I intend to get one), so I thought I might put it under the broiler for a minute, but as it turns out, the frosting never achieved the right stiff consistency. I’m now wondering if I should replace my cream of tartar – how long does that stay good for, anyway? It didn’t matter much, because it was absolutely delicious! I definitely recommend making a frosting, because you should know that the top of this cake was a bit off-putting without it, all craggy and stuff. Frosting is very forgiving for this kind of thing, all the more so when it’s marshmallow frosting!

For the cake
1 ½ lbs. sweet potatoes (about 2 to 3 medium or 2 large)
2 cups (250 g.) all-purpose flour
1 ¾ tsp. baking powder
½ tsp. baking soda
½ tsp. table salt
1 tsp. ground cinnamon
¼ tsp. (more to taste) ground ginger
2 pinches (more to taste) ground cloves
½ cup (1 stick) lactose-free butter or margarine, at room temperature
1 cup (190 g.) packed light brown sugar
½ tsp. vanilla extract
2 large eggs

For the frosting
3 large egg whites
¾ cup granulated sugar
1 pinch of salt
¼ tsp. cream of tartar (will help stabilize egg whites, don’t worry if you don’t have it)
½ tsp. vanilla extract

For the cake
Preheat the oven to 375 °F. Prick potatoes all over with a fork. Rest on a baking sheet. Roast for 45 minutes to 1 hour, turning once or twice, until soft. Let cool completely. Peel the sweet potatoes and run the flesh through a potato ricer or mash until very smooth; try not to use a food processor for this. Measure out 1 ½ packed cups of sweet potato purée (about 12 or 13 ounces) for the recipe. The purée can be kept in fridge for up to 3 days.

Preheat oven to 350 ° F. Line the bottom of an 8- to 9-inch square pan with parchment paper, then butter the paper and sides of the pan. (I used an 8-inch square pan, and I like to grease the pan even before putting in the paper because that helps it stick in place.)

Whisk flour, baking powder, baking soda, salt and spices together in a medium bowl.

In a large bowl, beat butter and brown sugar together with an electric mixer until light and fluffy, about 2 to 3 minutes. Add vanilla and eggs, and beat until just combined. Mix in sweet potato purée, then stir in dry ingredients just until they disappear.

Spread batter in prepared pan, and bake cake until a toothpick inserted into the center comes out clean, about 35 to 40 minutes. Let cake rest in pan for 5 minutes on a cooling rack, then invert onto cooling rack, and let cool completely.

For the frosting
Place egg whites, granulated sugar, a pinch of salt and cream of tartar in a heatproof bowl set over a saucepan of gently simmering water. Whisk mixture for 3 minutes, until whites are warmed and sugar granules feels mostly dissolved. Remove bowl from top of saucepan, then, with an electric mixer, beat egg white mixture on high speed until stiff, glossy peaks form, about 4 to 7 minutes longer. (Mine never did achieve stiff peaks. I’m not sure if my cream of tartar expired, or if perhaps the sugar hadn’t completely dissolved.) Add vanilla and mix until combined.

If you’d like to make huge, marshmallow-like dollops, spread a bit of frosting thinly over cooled sweet potato cake. Then, using a very large round piping tip or the corner snipped off a freezer bag, pipe large dollops of frosting all over thin frosting layer. If you’d like to skip the dollops, you can just spread the frosting, thick and swirly, all over. (Mine was more runny than thick, but delicious nonetheless.)

Using a kitchen torch, lightly brown the dollops so that they look (and smell!) like toasted marshmallows.

Keep the cake at room temperature for up to two days, or refrigerate it if you’re keeping it longer.