Cover approx. 20 tin cups used to steamed the cakes with parchment papers then put them into a steamer pot with boiling water.
Mix egg yolks, egg white, baking soda, sugar and vanilla until they make a smooth batter then put in turns, ½ portion of the flour, ½ portion of the soda water until all of them are used, mix well.
Take 5 tablespoons of the batter and put it in a separate bowl. Pour in the cocoa and mix well.
Pour the white batter into the tin almost to the top then pour in 1-2 teaspoon of the cocoa batter on top. Do this until all the batter is used up, 20 tin cups.
Close the steamer lid, let it steamed for 20 minutes with high temperature. Then use low temperature for another 5 minutes.
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