Directions

1. To roast sweet potatoes. Heat oven to 375 degrees. 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. Can be kept in fridge for up to 3 days, if baked in advance.
   
2. To make the cake. Heat oven to 350 degrees F. Spray the bottom and sides of an 8- to 9-inch square pan with nonstick spray.
   
3. Peel cooled sweet potatoes and run flesh through a potato ricer, or mash until very smooth. (Do not blend in a blender or food processor.) Measure 1 1/2 packed cups (about 12 to 13 ounces) from sweet potato mash; you may have a little extra.
   
4. Whisk 2 cups flour, 1 3/4 teaspoon baking powder, 1/2 teaspoon baking soda, 1/2 teaspoon salt and spices together in a medium bowl. In a large bowl, beat 1/2 cup butter and 1 cup brown sugar together with an electric mixer until light and fluffy, about 2 to 3 minutes. Add 1 teaspoon vanilla and 2 eggs, and beat until just combined. Mix in sweet potato puree, then stir in dry ingredients just until they disappear.
   
5. 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. You can speed this up, in the fridge.
   
6. To make the frosting. Place 3 egg whites, 3/4 cup sugar, a pinch of salt and 1/4 teaspoon 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. Add 1/2 teaspoon vanilla and mix until combined.
   
7. To frost and decorate. 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. Using a kitchen torch, lightly brown the dollops so that they look like toasted marshmallows.
   
8. Serve at room temperature cut into squares. Cake keeps at room temperature for two days; any longer, keep it in the fridge.