Sift cake flour, bread flour, and baking powder together ; set aside. Sift cocoa powder; set aside. Preheat an oven to 200℃ / 392F. Put 3 egg yolks in a large bowl, and put 4 egg whites in another large bowl. Let the egg white sit in a fridge.

Let oil and milk warm each in hot water, on extremely low heat. Add granulated sugar to the yolk. Beat it with a whisk for 1-2 mins until whitish and heavy.

Pour the oil little by little, while stirring. Stir until smooth. (Make it emulsify.) Pour the milk little by little, while stirring. After adding all, continue to stir for a min. Set aside to cool down a little.

Take the egg white out from the fridge. Beat it lightly until watery. Whisk it with a hand mixer on high speed.

When it becomes white and fluffy, add granulated sugar in 2 parts. Whisk it to the soft-peak stage. (The peak should hang down deeply.) Whisk slowly on low speed for a min to smooth it.

Mix lightly the egg yolk mixture. Add dry ingredients to the mixture. Stir it lightly until powderiness disappears.

And then mix strongly and speedy 30 times until heavy a little.

Add the meringue in 3 parts. Stir well until smooth at 1st addition.

From the next addition (2nd / 3rd), mix the top lightly, and then scoop it up and let it slip through your whisk. Repeat until smooth.

Use a spatula to scrape the edge and bottom batter off, and fold it into the batter. You should mix until the batter flows (2nd photp). Transfer 180ｇ / 6.3 oz of the batter into a bowl.

Add the cocoa to the transferred batter. Fold it until powderiness disappears.

Alternately put each dough into the pan. (Put each in 5-6 parts.) Scoop up the batter from bottom to top with a spoon. Go around the tube once. Scoop once at each place. If you mix too much, the pattern will be blurred.

Hold the tube and drop it lightly twice to remove big bubbles. Bake it at 170℃ / 338 F for 45 mins. Drop the pan lightly twice, and then upside down immediately. Let it cool completely (over 4 hours).

Remove the cake from the pan. Use your hand to push down the side and center (around the tube) to half the height. Please push little by little so that the cake doesn't tear.

Upside down, and push the cake out from the pan while rotating and pressing the bottom.

Peel off the cake from the bottom plate while pushing the cake little by little.

Done!!