This is meringue, which helps control the amount of expansion when baking a cake. I wrote this for those who aren't sure how to make meringue.

If any egg yolks get into the whites, whipping it will be really difficult, so please be careful not to add any yolk.
After adding the sugar, it will become harder to whip up, so if you're doing this by hand, I think it's better to add the sugar later. Recipe by -mimi-