Purple Sweet Potato Cake

This sweet potato cake is inspired by a similar cake from a Korean cookbook that I recently purchased. The concept of sweet potato cake sounds quirky to me, and of course, I couldn’t resist giving it a go. The sweet potato cake, known as Goguma cake, is apparently very popular in Korea. It is an Asian style soft sponge cake with sweet potato paste filling, covered in whipped cream and sponge cake crumbs.

I could have used the recipe from the book, but on closer inspection at the photo, it was obvious that they have missed out a couple of ingredients (and failed to mention a few steps).

My take on the Goguma cake is a Japanese style shortcake topped with a Mont Blanc inspired puree sweet potato cream. The sweet potato used in the Goguma cake is typically the yellow flesh type. However, I used purple sweet potato because I think that we can do with more purple food in our life. No, not really. It just so happened that there were purple sweet potatoes in the market, and making a purple cake seems like a good idea.

I’m pleased with how good the cake is. The sweet potato is not at all overpowering as I thought. It gives the cake a subtle flavour and sweetness. And with about one medium size sweet potato in this cake, it makes this a healthy cake enriched with antioxidant, vitamins, minerals and dietary fiber. That's what I'm telling myself anyway...




Purple Sweet Potato Cake

Makes 14cm x 28cm cake, serves 4

For the sweet potato topping:
1 medium size purple sweet potato
30 grams milk

For the sponge
Part A- egg yolk mixture:
3 egg yolks
30 grams caster sugar
60ml milk
1 teaspoon vanilla extract
40ml vegetable oil
80 grams cake flour
Part B- meringue:
3 egg whites
30 grams caster sugar
1 teaspoon cornstarch

For the sugar syrup
10 grams caster sugar
20ml hot water

For the filling
200 grams heavy (double) cream, chilled (Note: do not use low fat cream.)
20 grams caster sugar
1 teaspoon vanilla extract

To make sweet potato topping:
Steam or boil sweet potato until very tender. Peel skin and cut sweet potato into smallish pieces. Weigh out 200 grams of the cooked sweet potato, and reserve the rest for the filling and decoration. In a food processor, puree the 200 grams sweet potato and milk until smooth. Set aside to cool completely.

To make Part A- egg yolk mixture:
Preheat the oven to 170 degrees celsius (or 150 degrees celsius fan-forced). Line a 28cm x 28cm flat square cake pan with baking paper.

Using an electric mixer, beat egg yolks and sugar on medium-high speed until pale and creamy.

Combine milk, vanilla extract and oil in a small bowl. Add to beaten egg yolk mixture and whisk at low speed until just combined. Sift flour over egg mixture, and whisk at low speed until smooth and well-combined.

To make Part B- meringue:
Combine sugar and cornstarch.

Clean and dry the whisk thoroughly, and in a clean bowl, whisk the egg whites until foamy. Add half the sugar-cornstarch mixture and continue beating for a few minutes, then add remaining sugar-cornstarch mixture and beat until egg whites are glossy and stiff peak form.

Add one-third of meringue into egg yolk mixture from Part A. Fold in lightly with a spatula to loosen the mixture a little. Then add the rest of the meringue and fold to incorporate completely.

Pour batter into the prepared pan, spreading the mixture evenly and smoothing the top with a spatula. Lightly tap the bottom of the tray on the work surface.

Bake for 14 minutes until the cake has risen slightly and feels springy to the touch. Remove from oven. Turn sponge straight from oven onto a sheet of baking paper. Peel off baking paper from the bottom of the sponge. Cover with baking paper, then a tea towel. Set aside to cool completely.

To make sugar syrup
Dissolve sugar in hot water. Set aside to cool.

To make filling:
Combine cream, sugar and vanilla extract in a clean bowl. Using an electric mixer, whip cream at medium speed until stiff peaks form but cream is still smooth.

Weigh out 30 grams of whipped cream and whisk into cooled sweet potato topping.

To assemble:
When the cake is completely cool, cut cake in half so that you get 2 pieces of 14cm x 28cm cake (if you are using 28cm x 28cm cake tin).

Brush sugar syrup on one side of cake layers. Spread cream filling onto cake, then topped with sweet potato pieces. Place second layer of cake over and press down lightly. Pipe sweet potato filling over cake. I used a plain nozzle with a smallish hole and piped filling diagonally over cake. Decorate with parsley and a piece of sweet potato. Refrigerate for at least 2 hours before serving.