bakingcake
Not sure why this keeps happening. I read that mixing the yolks is critical and I mix until the color becomes whiter.
In the oven it grows very tall roughly 20 minutes after start, but starts collapsing after that. Once taken from the oven and put upside down it still collapses onto itself and becomes at roughly the same height as the pan
Chiffon cake, like it's cousin angel food cake, is mostly air. A big pile of protein bubbles stiffened with a little starch.
One very important step is not reflected in your recipe:
When the cake is completely baked the proteins have set and the starches have gelatinized but the starches are still very soft. The cake won't have its firm structure until the starches have been able to cool and set.
All recipes call for inverting the cake immediately when it comes out of the oven. The cake is allowed to cool, inverted and still in the pan, for a good hour to ensure the starches have set.
Special pans with legs or a long tube center are used for this:
Notice that in addition to the feet that it is a tube pan. This is important because the interior of these cakes is very insulated and won't be able to cook completely before the outside is overcooked.
When my angel food cake pans were packed I had good success using a pot with a mason jar in the middle. I was surprised it came out perfectly.
TL;DR -
Use a cake pan with a tube,
Invert the cake right when it comes out of the oven and let it cool completely.
*Note
Catija is correct that these amounts look way too small for a full cake. If you are trying to bake that in a full sized cake pan it could possibly be over rising and not have enough structure to support itself when it comes out.
I was just looking into trying this myself (after a disastrous attempt at bundt cake - tasted fine but broke in half coming out of the pan) anyway, I found a great site with lots of tips https://www.justonecookbook.com/how_to/perfect-chiffon-cake/ but what I also found, were other chiffon cakes where this had happened, and rather than it being a failure, people would decorate the groove with fresh fruit and icing sugar. So as long as it tastes good, just embellish it.
Best Answer
This will happen if you grease your cake pan or if the cake isn't allowed to fully cool before being removed from the pan. Another possible culprit is overbeating your egg whites.
Egg foam cakes, like chiffon, blow up with steam as they bake. The tube pan gives them more surface area both so they bake in the middle and also to give them more to cling to.
After they bake the cake is inverted until the egg proteins set. If not inverted, the cake will collapse as the steam escapes. A good rule of thumb is about an hour rest but the cake should be nearly room temperature.
If the tube pan is greased then the cake will not adhere near the end of cooking or through the inversion rest period.
The consistency of the egg whites is also important. If the whites are overbeaten they will lose some of their elasticity and give up their steam when they are stretched too far. Chiffon cake recipe do call for firm peaks, that is if you pull your beaters out the resultant peaks don't droop. But if you beat much past that phase the whites can get fragile.
Keep in mind that you should expect the cake to shrink slightly from it's max height when the steam is hot, but nothing like the collapse in your photo.
I don't believe that over or under beating the egg yolks would have caused the behavior you were seeing.