Put the egg whites in a bowl, gradually add the sugar in 2-3 batches, and whisk until it produces a stiff meringue.
In a separate bowl, combine the egg yolks and sugar, and whisk until pale and thick.
In the egg yolk mixture from Step 2, add milk and vanilla oil and mix with a whisk, then add the joshinko and mix.
Briskly stir the egg white mixture from Step 1 in 3 batches into the bowl in Step 3. Gently stir from the bottom of the bowl without reducing the froth.
Pour the batter into a mold, then tap the mold on a table to release air pockets.
Bake for 30 minutes in an oven preheated to 160℃. After 7-8 minutes into baking, make 4 shallow cuts into the top of the cake. This will help it to rise evenly. Once it's finished baking, turn the mold upside down and cool.
Story Behind this Recipe
Since I'm always eating sweets and bread made of wheat flour, I figured I should eat some rice flour, too.
Unlike wheat flour, rice flour is very dry and sandy, so there's no need to sift it. As long as you whip the egg whites to a stiff meringue, the cake will rise without any baking powder. The joshinko with give the cake a moist, fluffy, and chewy finish.