Sponge Cake recipe

With 4 eggs:
A) 4 eggs yolk, 40g sugar, 60ml oil, 100g flour, 1/2 teaspoon salt, 1/2 teaspoon soda, 45g orange concentrate, 45g milk, oranges zest

B) 4 eggs white, 100g sugar, 2g vinegar

Mix A together using hand beater

Mix B together using electric egg beater until the whole mixture set.

Mix B to A

With 5 eggs: 

A) 5 eggs yolk, 40g sugar, 70ml oil, 120g flour, 1/2 teaspoon salt, 1/2 teaspoon soda, 45g orange concentrate, 45g milk, oranges zest

B) 5 eggs white, 100g sugar, 2g vinegar

With 6 eggs: 

A) 6 eggs yolk, 40g sugar, 75ml oil, 150g flour, 1/2 teaspoon salt, 1/2 teaspoon soda, 45g orange concentrate, 45g milk, oranges zest

B) 6 eggs white, 100g sugar, 2g vinegar

References:

http://www.meishichina.com/Eat/Nosh/200807/40406.html
http://singmei1218.blogspot.com/2009/04/blog-post_2139.html