For the sponge cake: Line a 21 cm round cake pan with parchment paper. In a separate bowl, sift the matcha and cake flour 2-3 times.
Add sugar in 3 batches to the eggs, put the bowl over a pot of simmering water and whip with an electric hand mixer set to a high speed. Whip until bubbles form and remove when the mixture is warm to the touch.
Beat the mixture at a lower speed once it becomes heavier and starts to thicken.
Stop mixing once the mixture thickens and forms ribbons.
Sift the powdered ingredients into the bowl in three or more batches and cut through the batter with a spatula. Do not over mix.
Microwave the butter and milk in a heatproof container for 30 seconds until the butter melts.
Add a scoop of the mixture into the milk butter from Step 6 and mix well.
Gradually add the ingredients from Step 7 to the ingredients from Step 5, mixing it well with the spatula. Scrape up the ingredients from the bottom of the bowl (do not knead).
Pour the mixture from Step 8 into the cake pan, drop the pan slightly to remove bubbles.
Bake for 17-20 minutes at 340°F/170°C in the oven. Poke a bamboo skewer into the center of the cake. If it comes out clean, the cake is ready.
Drop the cake from a height of 20 cm once to prevent it from shrinking and flattening. Once it cools down, remove it from the baking sheet.
Using cling wrap, plastic bags or a wrung out wet cloth, wrap the cake with the browned bottom facing down.
Once the cake is completely cool, match the cake up to the size and shape of an 18 cm springform pan to cut it out. Place the cake on a plate with the browned part at the bottom.
Let's prepare the white chocolate mousse first. Soak the gelatin in water.
Whip egg whites until stiff peaks appear.
Whip the fresh cream (250 ml) until it is soft and creamy (not too much!)
Take out 100 g for the matcha mousse and chill in the refrigerator.
Heat the milk in a pot. When it starts to bubble at the edge, turn off the heat and melt the white chocolate.
Add the gelatin from Step 14 to the white chocolate mixture in Step 18 and dissolve everything. If it doesn't dissolve, warm it up again over low heat. (But make sure it's not too hot!)
Add the mixture from Step 19 to the cream from Step 16 (150 ml) and mix well.
Add the egg white (whipped) from Step 15 to the mixture from Step 20. Mix it well but do not break the foamy texture of the whipped egg white.
Pour the mousse on top of the sponge cake from Step 13, smooth out the surface and chill in the refrigerator.
For the matcha mousse. Soak gelatin in water.
Add egg yolks and sugar into a bowl and whip until it becomes thick and white.
Add the matcha and hot water into a pot and mix well. Add milk and dissolve the matcha over low heat.
When the edge of the green tea mixture starts to bubble a bit, turn off the heat and add the gelatin from Step 23.
Add the hot green tea mixture from Step 26 into the egg yolk mixture from Step 24 and mix well.
Add the egg and green tea mixture from Step 27 to the chilled fresh cream from Step 17 and mix.
Pour the green tea mixture from Step 28 into the mold with the chilled mousse from Step 22 and chill for another 2 hours in the refrigerator until it is set.
Once the mousse is set, sift matcha on top with a tea strainer to decorate.
Removing the cake from its mold will be easier if you wrap the mold with a warm and wet towel.
Here I sliced a 19.5 cm square sponge cake into two pieces, and then used a 5.5 cm cake ring to cut them again into this shape. I also used mousse film around the cake to hold it together. When you cut it out like this, you get 13 portions.
Story Behind this Recipe
This cake delivers a combination of green tea and white chocolate that tastes like Green Tea Latte.
You can also use whatever green tea sponge cake you'd like the mousse is delicious on its own. Using a "chasen" (tea whisk used in Japanese tea ceremony) during Step 25 will make the mixing easier. If you are decorating the cake with whipped cream, remember that you are using the sifted green tea powder to dust the cake.