Before you thought that I have just made a perfect chiffon cake, look at the photo below...
This is the sunken part that I have cut away before taking photographs. Haha!
Wait, I shouldn't sound too happy. This has been the Nth sunken chiffon cake that I have made. My previous orange, lemon, cheese and pandan flavoured chiffon cakes had all failed. All of them had the same problem of a sunken top after cooling. Everything was fine during the preparation, including beating the egg whites. The cake rose nicely in the oven, but when I unmoulded it from the pan, I always ended up in great disappointment.
Rei's Chiffon Cake Tips and Trouble II Tip #4 states that:
"If the cake's top (overturned, previously it was the bottom) is sunken in (like a small groove), the oven temperature is too high. If the oven temperature is too high, it will cause the cake to over-expand. So when the cake relaxes after cooling, it will cause this effect."
I might try stepping down the temperature to prevent the oven from getting too hot when making chiffon cakes again.
Strangely, my first few chiffon cakes did not have this problem. Is it me or my oven getting worse?
Recipe from Peng's Kitchen.