• Preheat oven to 170°C (338°F). Position the oven rack on the lowest in the oven. Have a 20-cm (8-inch) chiffon tube pan ready. DO NOT GREASE.
• Sift the dry ingredients: Sift together cake flour, baking powder, and salt. Separately, sift the cocoa powder.
• Beat the egg yolks: Using hand-held beaters or a stand mixer fitted with a whisk, lightly beat egg yolks on medium speed for a few seconds. Add sugar and continue beating until the yolks become pale, thick, and glossy. This will take about 3 - 5 minutes. Wash and dry the whisk attachment for the next step.

Make the meringue:

• In a large and grease-free mixing bowl, combine egg whites, cream of tartar, and a teaspoon of sugar. Whip on medium speed until the egg whites become thick and cloudy. Note: If using handheld beaters or a hand whisk, keep beating in one direction only, i.e. clockwise or anti-clockwise.
• Add the caster sugar in 2 to 3 lots, and continue whipping until the meringue becomes smooth, glossy, and form stiff peaks when the whisk is lifted. This may take 5 to 7 minutes, depending on your mixer and speed.

Make the vanilla cake batter:

• Add a scoop (about 1 cup) of meringue to the yolk mixture and mix well until the mixture is very smooth.
• Fold in the meringue and flour mixtures: Then fold in ½ of the remaining meringue with a rubber spatula or hand whisk until well incorporated. Next, gently fold in ½ of the flour mixture until well combined. Then repeat folding with all the remaining meringue, followed by all the remaining flour mixture until well combined.
• Fold in the liquids: In a small bowl, combine oil, milk, and vanilla extract. Stir until the mixture becomes a smooth emulsion. Fold into the cake batter in 2 to 3 additions, each time mixing into the batter gently without deflating the trapped air bubbles and until well incorporated.

Make the cocoa cake batter:

• Scoop out 250 grams of the cake batter into a separate bowl. Gently fold in the cocoa powder until well mixed.

Fill the pan:

• Make the first layer by alternating the vanilla and cocoa batters twice, so you have 2 of each batter. The vanilla portions should be slightly larger than the cocoa.
• Start the second layer by covering the vanilla batter with cocoa batter, and covering the cocoa batter with the vanilla batter. Repeat this alternating pattern for the third layer. 
• Make the fourth and final layer by covering the previous layer with all the remaining vanilla batter. Smoothen the top. If you're left with just a bit of cocoa batter, drizzle it on top.

Marble the batter:

• Using a metal spoon, gently scoop up the batter from the bottom of the pan to the top like the letter 'U'. Go around the chiffon pan once, no more than twice, and only in places you might have missed the first time around. Do not overdo it, otherwise, the pattern won't be distinct or clear.

Bake and cool:

• Bake for 45 to 50 minutes, or until done. DO NOT open the oven door. Check about 5 minutes before the end of baking, to test for doneness. When a skewer inserted into the centre comes out clean, the cake is done. Note: Cracking on the surface of the cake during baking is to be expected!
• Once the pan is removed from the oven, invert immediately and let it 'hang' until the pan is warm to the touch but not too hot to handle, at least 20 minutes.

Remove the cake:

• Remove by hand for smooth sides: Watch the hand technique in this video. Press down your hand gently on the cake to nudge it away from the side of the pan. Keep nudging bit by bit, working around the edges of the cake. 

  Then repeat, this time nudging more of the cake away from the sides but deeper down the pan. Keep repeating until you've released the cake totally from the sides of the pan. 

  Then invert the cake pan and gently press on the base until it comes loose. Finally, gently press the cake away from the funnel to release it.
• Remove with offset spatula or knife: Run an offset spatula or sharp knife gently around the sides of the pan, pressing against the pan as much as possible.

  Then invert the pan again so that the bottom is now facing up. Gently tap or push the pan’s base to loosen the cake from the sides of the pan.

  Then run the spatula or knife around the base to release the funnel. Chiffon cake is served ‘upside-down’.