BLACK SESAME ZEBRA COTTONY CAKE [OGURA CAKE]

Came across a Japanese Cotton cake recipe using black sesame powder.  Out of curiosity, I tried this cake but made it into a zebra pattern.  Since I do not have the black sesame powder, I used ground toasted black sesame seed [not fine enough].  I find it a bit difficult to blend the paste with the batter, that's why  certain parts have a big patch of black sesame due to uneven mixing [I was afraid to deflate the meringue due to over-mixing].   Next time, I must really grind the black sesame into a fine paste or get the powder...
Overall the cake tasted good and the texture is still cottony soft and moist.  The cake doesn't shrink much.

Ingredients
[8 or 9 inch square baking tin]
5 egg yolks
1 egg
45 ml corn oil
70 ml milk
65 gm cake flour  - sifted - set aside
1/4 tsp salt

50 gm black sesame powder [you can grind toasted black sesame seeds into paste]

5 egg whites
100 gm sugar [I reduced to 80 gm]
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 170 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 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.  Remove 1/3 of the cake batter to another bowl.  Add in the black sesame powder, mix well.
7. Scoop the 2 batters alternately to create the zebra effect of the cake into prepared cake tin [using a small ladle, put the plain portion in the centre of tin and top with black sesame batter, shake the pan lightly and the batter will spread out itself].  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 10 minutes, then lower temperature to 140-150 degrees C and continue to bake for 50-55 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.