Japanese Sweet Potate Chiffon Cake

Japanese sweet potato is one of my favorite winter snacks. There's countless possibilities with this sweet veggie, you can caramelize, bake, boil, mash, anything!
Today I tried to use sweet potato puree to add to my all time favorite cake recipe - chiffon cake.
The recipe calls for a 12 cm chiffon cake mold.

Ingredients:
Egg whites 180g (~6 large eggs)
sugar 90g
corn starch 10g

Egg yolks 70g (~4 large eggs)
water 80g
canola oil 70g
mashed sweet potato 100g
all purpose flour 90g
caramelized sweet potato (diced) 150g

decoration
whipping cream 200g
sugar 18g
contreau/rum 1 tbsp
caramelized sweet potato optional

Steps:

1. Preheat oven 320 F (160 C) 
2. Caramelized sweet potato: Cook sweet potato in 100g water, 40g sugar and 1 lemon zest until softened. Diced 150g and mashed the remaining for use. Add 1 tsp of rum to sweet potato if desire. 

1. Chill mixing bowl, whisk egg whites in low speed using electric mixer until bubble rises. Then mix in high speed. Add all sugar and whisk until soft peaks form. Add corn starch until peaks stiffen.
2. After stiff peaks are formed, slightly stir 3-4 times manually and test the consistency of the egg whites

1. Use a different mixing bowl, blend egg yolks, water, oil until smooth and thick. Add in smashed sweet potato, mix well.

1. Add all purpose flour, stir in the same direction until combined. Don't over mix.
2. Gently fold in diced sweet potato into the mixture. 
3. Put 1/3 of egg whites and mix into (5), gently stir and combine.
4. Then pour (6) into the remaining egg white mix, gently fold in the batter without breaking the bubbles from the egg whites.

1. Lightly pour batter into cake mold, gently tap the mold onto table to even out the surface. Bake for 45-50 minutes or until golden brown
2. Prepare cream by mixing whipping cream, liquor and sugar. 
3. After cake is ready, gently tap the mold on the table again, then flip mold upside down. Place the mold on table until completely cool. Remove the cake by cutting the cake along the edge, lift up the cake by supporting the detachable base. 

1. Cream the cake with cream and decorate with sweet potato as desired. Enjoy!

Tips:

1. The consistency of the egg whites is a major key to success. NEVER over mix or under mix. Make sure it stays in the bowl even you tilt it. That shows the whites are stiff enough to rise when baking
2. measure ingredients precisely to ensure the cake is not too moist or dense
3. Don't over heat the oven to speed up the process. If the cake is over-heated, it will sink down when it cools down
4. too much wet ingredients will also make the cake sinks