Cover approximately 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 Tbs 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 tsp 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.