Line the cake pan with parchment paper. Preheat the oven to 160°C.
Crack the eggs into a bowl and beat well. Add the sugar as you mix.
Place the bowl over a double-boiler as you whisk, and heat the egg mixture until warm (about 40°C). (I do this over open heat, but use a double-boiler if you are not confident).
Remove from the double-boiler and beat with an electric mixer.
It is beginning to get fluffy in this picture, but this is not enough. Keep beating so that the texture becomes smooth.
I prefer to beat well, so I beat until it becomes smooth and fluffy.
Beat until the batter forms ribbon-like when you lift the mixer and let the batter fall back into the bowl. Then turn down the speed to low and mix for another 30 seconds to even out the size of the bubbles.
Sift the flour into the batter.
Using a rubber or wooden spatula, fold the flour into the batter from the middle to the out side, scooping up from the bottom of the bowl (pink arrow). At the same time, turn the bowl towards you with other hand (yellow arrow).
This picture shows you when the flour is folded into the batter evenly, but if you stop here, the cake turns out with a less pleasing texture, so continue mixing for a bit longer.
The batter is now glossy, so you can stop mixing. The cake will turn out rubbery if you mix too much!
Pour into the baking pan. If you mixed the batter until it gets glossy, you will get a smooth, moist cake. If you did not mix enough, you will get a dryer, less smooth cake!
Drop the pan lightly around 3 times from a height of about 5 cm to remove any large bubbles.
Bake in the preheated oven at 160°C for 30-40 minutes. It is done when nothing sticks onto a skewer (wooden or bamboo) when you insert it into the cake (I baked for 35 minutes).
Drop the cake pan once from a height of about 20 cm, then remove from the pan. Place the cake on a wire rack and cover loosely with moistened kitchen towel or plastic wrap to cool.
Once it has cooled, slice to your desired thickness! If you want to make the top of the sponge cake flat, place a piece of kitchen paper or plastic wrap on the wire rack, and place the cake upside down to cool.
[For a 15 cm cake pan] 110 g eggs, 50 g sugar, 50 g cake flour. Weight of the eggs does not include the shells, about 2 eggs' worth.
[For a 21 cm cake pan] 240 g eggs, 110 g sugar, 110 g cake flour. Weight of the eggs does not include the shells, about 5 eggs' worth.
If you use a cake pan that is not 18 cm wide, bake at the same temperature, but adjust the time accordingly. If the pan is smaller, shorten the baking time; if it is bigger, bake for longer.