How to make the world's softest chocolate sponge cake | the perfect castella cake with no cracks

Firstly, castella is more delicate than a typical Western sponge cake. Though the ingredients are similar, mainly focused around flour, sugar and eggs, castella cake tastes quite different.

[Ingredients]
5 eggs
80g cake flour
25g cocoa powder (unsweetened)
130g milk
cooking oil 65g
1g salt
110g sugar