Japanese Cotton Sponge Cake - How to stop collapsing? I've recently found videos about Japanese "Cotton" Sponge Cake.  (Possibly also called a 'Jiggly Cake'?)  I've never had it, but I figured it looked good, so why not try it?  All of the videos I have seen list ingredients, and demonstrate technique, but don't necessarily explain the techniques.  I feel I have missed something.  In particular, 2 of my 3 sponges collapsed very shortly after they come out of the oven.  My three variations all used the same recipe, but had varied baking technique.  Here's the recipe I've been using:

Ingredients:


50 mL canola oil 
80 g cake flour 
1/4 tsp salt 
1/4 tsp baking soda 
90 mL half and half * 
1 tsp vanilla extract 
1 egg 
5 egg yolks 
5 egg whites
1/2 tsp cream of tartar 
80 g sugar


Method:


Preheat oven to 350 F.
Heat canola oil in a pan. **
Whisk together flour, salt, and baking soda in a medium pan.
Add the oil to the flour mixture and stir until smooth.
Add the half and half and vanilla to the mixture and stir until smooth.
Add the egg and egg yolks to the mixture and stir until smooth.
Add the egg whites and cream of tartar to the bowl of a stand mixer, stir.
Whip the egg whites until foamy.
Slowly add the sugar while continuing to whip.
Whip to medium to stiff peaks (under is better than over).
Add 1/3 of the whites to the flour mixture and incorporate.
Switch to a spatula.  Add the remaining whites to the mixtures in two batches, folding gently to just incorporate.
Gently pour the mixture into an angel food cake pan, smoothing the top.
Bake for 50 minutes.
Remove from oven and drop the pan from about 6 inches onto a hard surface. ***
Cool upside down.


My two failed attempts involved using:


a water bath and a loaf pan (with parchment extending the walls) instead of an angel food pan
a steaming tray (half sheet pan with water) on the rack below the cake, and a springform pan (again with parchment).


Both of these attempts resulted in cakes that had not cooked all the way through the bottom. and collapsed within seconds of coming out of the oven.

Of the third attempt, I did get a nice, lightly, subtly sweet cake.  It's something like the texture of an angel food cake, but with more of a pound cake flavor (from the yolks, I assume).  Still, it loses volume, and I can't keep that nice domed top that I've seen in the videos.

Questions


Is collapse inevitable?  The Angel Food Pan retainted about 2/3 of its height. (But was at least cooked through)  How  can I keep maximum volume?
Is there something wrong with my technique or ingredients?  I'll point out: I've replaced 80 mL of milk with 90 mL of half &amp; half; I've added the cream of tartar because that is a standard technique I've learned when whipping egg whites; and I've added the baking soda to offset the acidity of the cream of tartar and to help with lift.
Is there a benefit to the steaming or water bath?  How should I use it without getting an uncooked center?


Footnotes

*The recipes have called for milk, but I don't keep it on hand.

**I have no idea why some of the recipes I have seen do this.  My next attempt will likely avoid heating the oil.

***Supposedly, this helps prevent collapse?  I've seen it in the videos, anyway.
Try using the actual recipe without substitutions or additions.
I've not made this type of cake, so I cannot answer with 100% certainty.  However, in general I would recommend not tinkering with ingredients the first time you make a recipe, especially when baking.  There are structural reasons for a lot of baking ingredients, and messing with them is a dangerous game.
In particular, fat content and raising agents are pretty important for cake structure, and it sounds like you've altered those right off the bat before making the cake once with its actual recipe.
Half and half has at least 3 times the fat of milk.  This could be having an impact on the cake's ability to rise.
I suspect the main culprit is the baking soda, though.  You've added baking soda and acid to a recipe that has neither, which can cause bubbles to expand and pop rather just staying put, which in turn causes cakes to collapse soon after they come out of the oven. (see this article for details)
The need for cream of tarter when whipping egg whites can be overstated.  Its main functions are to prevent overbeating and stop the foam from collapsing if it is sitting around after being beaten. It is pretty difficult to overbeat egg whites if you are whisking by hand.  If you're using an electric mixer, just keep an eye on it. Because you're only beating to medium-stiff peaks in this recipe and are then using the whites right away, the cream of tarter can be left out of the egg-beating process.
In conclusion, try the recipe as it is written.  Definitely remove the added baking soda and cream of tarter.  If you really have no access to milk, water is a better substitution when cake-baking than half-and-half is. If you really want some dairy in there, I might recommend 25 ml half-and-half + 55 ml water as a better substitution for 80 ml of milk than all half-and-half (this will have a much closer fat content to 80 ml of milk than 90 ml of half-and-half will).
Side note: if you ever do want to add cream of tarter to egg white to help with the beating, you do not have to then add baking soda or another base to balance out this tiny addition of acid.  Leave baking soda out of recipes that don't call for it.