PURPLE SWEET POTATO COTTONY CAKE [OGURA CAKE]

This cake is supposed to be purplish in colour but I think I added too little of the mashed purple sweet potato.  Anyway, it doesn't matter as with this quantity used, the cake texture was lovely [spongy firm] except without the purplish colour.  I guess you can try with any colour sweet potatoes, which I will be doing........

Ingredients

[8 or 9 inch square baking tin]

[recipe from a food magazine with modifications]

5 egg yolks

1 egg

60 ml corn oil [I used 50 ml]

70 ml milk

65 gm cake flour  - sifted - set aside

50 gm mashed sweet potato [purple sweet potato]

1/4 tsp salt

5 egg whites
70 gm sugar [original uses 100 gm - too sweet]
1/4 tsp cream of tartar

1. Line base of a 8" square tin with grease proof paper.   I greased the sides, too and sprinkle lightly with a little flour [this way, the cake can be easily removed from pan after cooling].
2. Preheat oven to 160 degrees C.    Place a baking tray on the lowest rack, pour in enough water [about 1.5 cups] for steam baking, then place a baking rack over it. 
3. Whisk egg yolks and 1 whole egg until creamy then gradually drizzle in the corn oil followed by milk.    Whisk until well combined.
4. Fold in sifted flour and sweet potato paste, stir until batter is smooth.  Set aside.
5. Use a cake mixer, whisk egg whites until frothy, then add in cream of tartar.  Beat until soft peaks then gradually add in the sugar in 3 batches and whisk until stiff peaks formed.  Do not over beat egg whites otherwise it may be difficult to fold in to egg yolk mixture.
6. Fold in 1/3 of the meringue into the egg yolk mixture until combined.   Then fold in the rest of the meringue lightly in two portions until well combined. 
7. Pour the batter into prepared cake tin.   Tap baking tray on work surface a few times to release trapped air bubbles before baking.
   
8. Place it over the baking rack to steam bake cake in a preheated oven at 160 degrees C for 70 minutes [by then the water bath would have dried up].  
   
9. Remove cake and invert it to cool for about 5 minutes [otherwise the cake base may be moist because of vapour from steam-baking].   Remove baking tin and the paper linings immediately and leave cake to cool on a wire rack [base facing upwards].  The cake may shrink a little when cooled because of less flour used [low gluten cake].
   
   
10. Turn the cake over when completely cooled.  Refrigerate before slicing if preferred.