My wife has been working on making a chiffon cake. The picture below is attempt number 10. She has varied the temperature of the oven, used a metal tray or no metal tray in the oven, and other factors. The cake rises and looks ok from the top, but it always has this weird form on the bottom. To me, it seems that a steam / gas pocket is forming possibly because the heat is too hot on the bottom. She says she has lowered the bottom temperature and the results are similar.
My question is what causes this to happen and what are some solutions?