Mix the cake flour and green tea powder together. Preheat the oven to 180℃.
Put egg whites in bowl and whip with an egg beater until stiff, add sugar a little at a time while whipping, until the meringue turns shiny and stiff peaks form.
In a separate bowl, add the egg yolks, vegetable oil, and water, and blend with a hand mixer without forming bubbles. Add the dry ingredients and mix on low speed until it becomes glutinous.
Add 1 dollop of meringue from Step 2 into the cake mixture from Step 3 and blend together with a mixer.
Add the mixture from Step 4 to remaining meringue, and fold together until the white streaks disappear.
Pour the mixture into a cake mold and bake for 30 minutes at 180℃. When it's ready, turn it upside down and let it cool. Remove from the mold after it thoroughly cools to room temperature.
I tried adding large dainagon adzuki beans to the mix.