Sift the flour two or three times. Line the cake pan with parchment paper. If you lightly grease it with oil, it will stick nicely.
Preheat the oven to 170°C.
Beat the egg whites with an electric mixer on high speed. Add the sugar in 2 to 3 parts, beating well with each addition.
Once stiff peaks form, beat in one egg yolk at a time on high speed.
When it has all mixed together, beat for 1-1:30 minutes on low speed to smooth the batter.
Next, add the milk and mix lightly.
Then, add all the flour and mix together with a whisk. It should be thick enough to lift up and fall off the whisk. You can also use a spatula.
Stir 30-35 times until no longer powdery, it's evenly blended, and becomes glossy.
Pour the batter into the pan. Pour from a little high up so that air bubbles do not form in the batter. Lightly tap the edges of the pan 2 or 3 times, then bake at 170°C for 30-35 minutes.
If a skewer or toothpick inserted doesn't come out clean, bake for an additional five minutes while keeping an eye on it.
Once baked, drop onto a table from a height of about 20 cm to deflate.
Flip upside down onto a cooling rack and remove the paper. Cover with a moistened and tightly wrung kitchen towel and let cool.
Once completely cooled, place in a plastic bag and store in the refrigerator.
For a chocolate sponge cake, use 70 g of flour and 10 g of cocoa for the 18 cm cake, and 50 g of flour and 10 g of cocoa for the 15 cm cake.
To make a matcha cake, replace the cocoa with matcha powder (as in Step 14).
Story Behind this Recipe
I was making my own decorated roll cake recipe, so I wanted to make my own sponge cake for use in roll cakes and regular cakes. It's not as tall as sponge cakes with butter in them, but it's fluffy.
When mixing in the flour, over-mixing will cause bubbles to form in the cake, preventing it from fully expanding, but if you under-mix, the cake will become crumbly. Try stirring about the number of times suggested here. After making it a few times, you can find the balance you like the best.