With an electric mixer, on low speed beat egg whites till frothy. Add in cream of tartar and continue to beat for a minute then add half the sugar. Beat till foamy and add rest of sugar, continue to beat till soft peak stage.
Add 1/3 portion of the merringue into the egg yolk batter to loosen and combine with a hand whisk.
Add the second 1/3 portion of merringue and fold in gently with hand whisk
Finally, pour the egg yolk mixture into the egg white merringue and fold gently with spatula till just combined.
Pour the batter slowly into chiffon cake pan to enable large bubbles to pop as the batter flows down. When done, even out top of batter with spatula, then tap pan on the countertop.
Bake in preheated oven at 170oC for 30 min. Then reduce heat to 150oC for another 15 min. Immediately remove from oven when done and cool cake pan upside down.
Let cool completely before unmoulding. To unmould, use a thin knife to cut carefully around the edges of the pan, then around the middle, lift out tube and cut cake from the bottom. All this is best done while holding the knife still and moving the cake pan instead as you will achieve a neater edge.