Whisk egg yolks at high speed for 10 seconds, then gradually add 3 tbsp warm water and 40g sugar while whipping for 3 minutes.

Gradually add vegetable oil while whisking. Add it slowly to prevent the foam from collapsing.

Whisk the egg yolk mixture until it leaves a trace when dropped.

Whisk egg whites at high speed for 1 minute, then gradually add 40g sugar to create a meringue.

Combine sifted cake flour, cocoa powder, baking powder, and meringue into the egg yolk mixture in thirds, folding gently.

Pour the batter into a chiffon pan sprayed with water, tap lightly to remove air bubbles, and bake at 170°C for 40 minutes.

Cool the chiffon cake upside down for about 1 hour after removing it from the oven.