Use pure cocoa powder with no added sugar. Don't use hot chocolate cocoa mix.
Combine the cake flour and pure cocoa powder in a small bowl and mix well. Line a cake pan with parchment paper and preheat the oven to 160℃.
Beat the eggs in a bowl, add the sugar and mix immediately.
Place the bowl into a hot water bath, continuously mix with a whisk, and heat until the egg mixture becomes warm (about 40°C). (I do this right on the flame, but feel free to use a hot water bath if you are worried about it.)
Remove the bowl from the hot water bath and whip with the hand mixer. The mixture becomes fluffy, but keep whipping until it has turned into a fine foam.
I like to whip the mixture really well, so I whip it until it turns into a fine foam and frothy.
Whip until the mixture forms ribbons which do not disappear when you drop it from the hand mixer. Finally, whip at a low speed for 30 seconds to make it fine and smooth.
Sift in the combined cake flour and cocoa powder from 2.
Use a rubber or wooden spatula to fold the mixture from the middle to the outside (pink line). It is easier if you turn the bowl towards you with your other hand at the same time (yellow line).
This is how the batter looks when the flour ingredients are blended in and it's no longer floury. But if you stop here then it will become a rough and gritty batter, so fold it a bit more.
If you mix it further after the flour ingredients have blended in, the batter will become glossy. When it looks like this photo, it's ready! It will be dry if you don't mix it enough...
Pour the batter into the cake pan from 1. ※ It will be dry if you don't mix it enough, and a springy batter if you over mix it, so get your own feel for it!
Drop the cake pan lightly on the counter a few times to remove air bubbles. (You can drop it from a height of about 5 cm)
Bake in a preheated oven at 160℃ for about 30~40 minutes. Check with a bamboo skewer, and if it comes out clean, it's done! (It took me 35 minutes to bake this cake.)
Drop the cake pan once from a height of about 20 cm. Remove the cake from the pan and place it on a metal rack. Cover loosely with a moistened kitchen towel or cling film and cool.
When it has cooled, slice to your desired thickness! When you want to make the top of the sponge cake flat, place a piece of kitchen paper or cling film on the metal rack, and place the cake upside down to cool.
Recipe ID: 1425955 "No Need for Chocolate! Chocolate Cream with Cocoa" goes really well with this cake
I made "For Kid's Day! Chocolate Banana Carp Cake (Recipe ID: 1426948)"
Story Behind this Recipe
I came up with this chocolaty cocoa sponge version when I made the sponge cake from Recipe ID: 1426499
Keep mixing the egg mixture continuously when heating, or the eggs may be cooked. So be careful! The key is to whip really well and fold in lightly! Don't fold too much as this batter with cocoa powder loses its air bubbles more easily than the plain one! The baking time varies depending on your oven.