Preliminaries: Line the mold with kitchen parchment paper. Leave the eggs in the refrigerator until you need them. Sift the cake flour and matcha together 3 times.
Put the egg whites in a bowl, and whip with a handheld mixer. (Make sure that you use an impeccably clean bowl. If it has any water or oil in it the egg whites won't whip up properly!)
When the egg whites are evenly whipped, add 1/2 of the granulated sugar and whip together. When it's very fluffy add the rest of the sugar, and keep whipping until it turns into a smooth, glossy meringue.
If stiff peaks form when you lift up the mixer, it's done!
Add the yolks to the meringue and mix well.
Add the sifted flour and matcha all at once.
Fold the flour into the meringue in a cut-and-fold motion, mixing up from the bottom. (Do not beat the batter.)
Put the butter and milk into a heatproof bowl, cover with plastic wrap and microwave for 60 to 80 seconds or so.
Mix to dissolve the butter. (Keep mixing until it cools down to about 50 °C.)
Add the butter-milk mix to the batter a little at a time. (If you add it all at once it will sink down to the bottom! Add it in 3 to 4 batches.)
Mix it in using a cut-and-fold motion, working rapidly. (Do not beat the batter!)
Pour the batter into the lined mold. (Since the batter at the bottom of the bowl does not rise well, pour it around the perimeter of the mold, where it will cook faster.)
Drop the mold from a height down on your work surface 2 to 3 times to eliminate any air pockets, and smooth out the surface.
Bake for 30 to 40 minutes in a preheated 170 °C oven. If you poke the middle with a bamboo skewer and it comes out clean, it's done! As soon as it's baked, drop it from a height of 20-30 cm onto your work surface to prevent the cake from shrinking.
Put a well wrung out moistened kitchen towel on your work surface, and take the spongecake out of the mold. Peel the paper off the sides. (Put the cake top side down.)
Put the mold back on the cake and leave for 2 to 3 minutes. (This is to make the top surface flat, and to even out the moisture in the cake.)
Take the mold off and peel off the paper from the bottom.
Turn the cake over, and cool on a cake rack.
Cover the spongecake with a moistened and tightly wrung out kitchen towel, and leave to cool.
When the spongecake is cool, slice horizontally and fill with the fillings of your choice, such as whipped cream. (The cake is easier to slice if you let it rest for a day. Keep in a plastic bag.)
How to slice the spongecake cleanly: Mark the sides vertically with a knife so that you can re-stack the layers later.
If you don't have a cake slicer: Insert toothpicks where you want to slice through the cake, starting with the upper layer, spacing the toothpicks evenly (in 4 to 8 places).
Slice through the cake on top of the toochpicks horizontally. Slice the lower layer in the same way.
Re-stack the layers, following the vertical marks you made in Step 21.
Tip: If you flip the lowest layer over, put the middle layer on top, and the top layer in the middle, the cake will be easier to decorate.
To make a 12 cm diameter cake: Re-calculate the amounts based on 1 egg. (I halve the amounts used for the 18 cm cake.) Bake for 25 to 30 minutes (28 minutes works best).
To make a 15 cm diameter cake: Halve the amount of ingredients used for the 20 cm cake. Bake for 30 to 35 minutes (170 °C, 34 minutes in a gas oven).
To make a 21 cm diameter cake: Re-calculate the amounts based on 5 eggs (Or multiply the amounts for the 18 cm cake by 1.5.) Bake for 30 to 40 minutes (35 minutes works best).
To make a 12 cm diameter cake plus a 15 cm diameter cake: Use the amount of ingredients specified for the 18 cm or 20 cm cakes (I prefer the latter). Baking times - 12 cm = 28 minutes, 15 cm = 34 minutes.
When trying to figure out how much batter you need for a particular mold, calculate for 1 egg worth of ingredients per 500 ml of capacity.
Story Behind this Recipe
I wanted to make a matcha sponge cake after making my milk and chocolate sponges.
Step 4: You must make a meringue that forms stiff peaks! Steps 7 and 11: After adding the flour and milk-butter, try to mix the batter as little as possible! Use a cut-and-fold motion! Step 20: I recommend spreading the cake with cream, then letting it rest for a day! It will become even more moist.