Are you familiar with the Korean sweet potato cake? It is a moist, soft and mildly sweet layered cake filled with sweet potato paste or cream inside. Unlike the sweet potato cake that Americans are more familiar with, there is no hint of spice but only a highlighted note of natural sweet potato flavor matched with soft, moist texture from the sponge cake and whipped cream frosting.
Korean sweet potato - more widely known as Japanese sweet potato in the U.S. - has red-colored skin with pale yellow flesh which turns into bright yellow once it's cooked. It's naturally creamy and sweet, so you can enjoy its full flavor after simply boiling or even cooking in a microwave until soft.
But to make it a feature ingredient in a cake, it requires a few more steps than just boiling it until soft. Actually, I'll warn you now that this is one of the most extensive recipes I have on my blog. If you follow my recipe of making this cake from scratch, it will be at least a half-day project. The steps are pretty easy, but everything needs to be cooked then cooled to room temperature or cold before proceeding to the next step. After cooking and assembling everything, the reward of tasting the cake requires another couple of hours to overnight of resting in the refrigerator while you let the cake flavors come together. And the resting part is an essential step that you shouldn't skip.
I made the basic sponge cake in a rice cooker, but feel free to bake your basic sponge cake in the oven. Google 'sponge cake recipes' and most of them will direct you to bake the cake in the oven. My recipe is determined by the rice cooker bowl, which determines the size of the cake (9 inch in diameter). From there I cut out a smaller round (in this case, it happens to be 5 inch in diameter) to determine the size of the actual sweet potato cake and use the trimmings as powdery garnish. There was a lot of sponge cake powder leftover, so I believe you can safely cut out a 6-inch round without adjusting the rest of the recipe.
The good news is that there are ways to speed up the process. You can use a simple store-bought sponge cake and follow the rest of the steps for cooking sweet potatoes, making sweet potato paste, and assembling the cake as specified in my recipe. You can also skip the last garnish part of piping out the whipped cream frosting and putting pistachios on.
If all else fails, I hope you find a Korean bakery near you and enjoy a slice of a sweet potato cake. It is most likely available in all Korean bakeries right about now.
Rice Cooker Sweet Potato Cake (고구마 케이크)
Yield - One 5-inch cake (6 generous slices)
INGREDIENTS:
Basic Sponge Cake in Rice Cooker
1 cup All-purpose flour
1/4 teaspoon Salt
1 teaspoon Baking powder
4 Eggs, whites and yolks separated
1/2 teaspoon Vanilla extract
1/2 cup Sugar
1/2 cup Any neutral-tasting vegetable oil (such as sunflower seed oil)
plus a few drops to grease the rice cooker bowl
Whipped Cream Frosting & Filling
1 1/2 cup Heavy cream
2~3 tablespoons Sugar
1/2 teaspoon Vanilla extract
Sweet Potato Paste
1~2 Japanese sweet potatoes (1 packed cup after cooking and mashing)
1/4 teaspoon Salt
1~ 1 1/2 cups Water
1 tablespoon Sugar
1 cup Whipped cream frosting
6~10 Pistachios for garnish
DIRECTIONS:
Basic Sponge Cake
1. Grease the rice cooker bowl with vegetable oil by drizzling a few drops of oil in the rice cooker bowl then coating the entire side of the rice cooker bowl with a piece of paper towel or a pastry brush.
2. In a bowl, whisk together to mix flour, salt and baking powder to combine. Set aside.
3. In a separate bowl, whisk egg whites. When egg whites turn opaque white and foamy, add 1/4 cup of sugar and continue to whisk. When sugar has been incorporated and the egg foam starts turning glossy, add the remaining 1/4 cup of sugar and continue to whisk. Stop when the egg foam (i.e., meringue) is firm and silky-glossy. When you lift the whisk, the tip of the egg meringue should gently fall, which is about soft-medium peak.
4. Add egg yolks and vanilla extract and whisk gently to incorporate. Add oil and whisk gently to incorporate until there is no separation between the egg foam mix and oil.
5. With a wooden spoon, gently incorporate 1/2 of the flour mix into egg meringue first. Add the rest and fold in gently just until there are no lumps of flour. Do not over-mix.
6. Pour the batter into the greased rice cooker bowl. Tap the side of the bowl to release big air bubbles. Place the bowl in the rice cooker.
7. Press 'Steam' (a.k.a., 'Multi-cook') for 40 minutes if your rice cooker has that option. Alternatively, 'Cook' twice, i.e., press 'Cook' and when it's done, press 'Cook' again. You might have to wait for the rice cooker to cool a bit before starting the second run in case of an older, simpler rice cooker.
8. When it's done, flip gently on a cooling rack. Let it cool to room temperature. Cover with plastic wrap and refrigerate to allow time to moisten and its flavors to settle before serving.
Whipped Cream Frosting
In a big metal bowl (1 1/2 cup of heavy cream should come up to about 1/3 or less of the bowl), add heavy cream, sugar and vanilla extract. Whisk heavy cream to whipped cream with a medium peak. Don't over-whip. Keep it refrigerated.
Sweet Potato Paste
1. Wash sweet potatoes, cut off both ends and cut each potato to 4-6 pieces of about equal size. Add 1 cup of water, salt and potato pieces in a pot, covered with a lid. Bring it to a boil then simmer until potatoes are completely soft, about 20~30 minutes. You may need to add another 1/2 cup of water while the potatoes are cooking if the water level becomes too low.
2. While sweet potatoes are still hot, peel and discard the skin. Add 1 tablespoon of sugar to potato flesh then mash it with a potato masher or a fork. You will have small pieces of sweet potato chunks, which is okay. Let it cool to room temperature.
3. Add 1 cup of whipped cream frosting and fold it completely in mashed sweet potato. Taste and adjust sweetness by adding more sugar if necessary. FYI, you don't need to mix in the whipped cream, but mashed sweet potato alone will feel very dense as a filling in a cake. Set aside.
Assembly
1. When the sponge cake cools to room temperature, trim off the outer brown layer and discard (in this case to my mouth). With any round mold, cut out a smaller cake base (see picture). I was able to get one 5-inch cake base and 2 cupcake-sized mini base cakes. The relative sizes of the base cake and the leftover depend on the rice cooker you're using. The rest of the steps in 'Assembly' will only discuss the 5-inch base cake.
2. With the leftover cake, crumble it through a coarse sieve to make sponge cake powder. Set aside.
3. Cut the base cake into 3 layers. On the bottom layer, spread out 1/2 of the sweet potato paste to an even layer. If you have a pastry spatula, it makes the job easier. Top the sweet potato layer with another cake layer. Spread the rest of the sweet potato paste to an even layer on the cake. Cover with the last cake layer.
4. Take out the whipped cream frosting from the refrigerator. If the frosting has become loose, whisk a few times to bring back to the whipped cream consistency. Set aside 1/2 cup of frosting aside for garnish. Spread the frosting all around the cake. It is easier to start from the top then down to the sides. Don't worry about making a perfectly smooth cake with the whipped cream frosting. Just make sure the cake is completely covered in generous amount of whipped cream frosting.
5. Sprinkle sponge cake powder from Step 2 under Assembly all over the cake. Make sure the cake is completely covered in sponge cake powder.
6. If you want the sweet potato cake look sold in Korean bakeries, add the reserved whipped cream frosting into a pastry bag with a pastry tip. Pipe out 6~8 dots on top of the cake and put a pistachio on top of each whipped cream dot.
It is best to leave the composed cake rest overnight in the refrigerator. It really does make a big difference in flavor. Enjoy!
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KOREAN WORDS
가을 (ga eul) autumn
고구마 (go gu ma) sweet potato
