Charles T, I too, feel your pain.
I will say, though, that when I look back on the entire trajectory of my baking projects, inlcuding the mistakes, what I see is the happy accumulation of skills and knowledge. And the mistakes contribute as much as, if not more, to my body of knowledge than the successes do!
I agree with Bill about the underbaking. I think I saw that you baked these in cupcake liners but not in a cupcake pan, perhaps the sides of the cupcake pan conduct heat better than air? Also, in general with genoise it is better to err on the side of a too-hot oven than a too-cold one.
I also agree with freshkid, this is a tremendous amount of butter for a sponge cake. There’s a huge difference between the 6 oz (for a 4-egg recipe) here and the 2.5 oz that Rose specifies in her genoise riche, or the 1.9 oz of oil she uses for 4 eggs worth of chiffon.
And for the record, this is a chiffon recipe using a simpler whole egg meringue instead of the traditional stiff egg whites. I believe the reason Sherry calls it a Genoise is because of the way she organizes this book, with master recipes grouped with a family of derived recipes.
Finally, sponge-type recipes do shrink when cooling, which is why some types are cooled upside-down in an ungreased pan, to minimize shrinking/falling.
So with all this in mind, here are my thoughts for what else to try (provided you have the will to go on…)
-If you are determined to follow the recipe as written, increase the heat in your oven and use a paper liner in a cupcake pan. Accept that they will not be rounded and go with it- fill any depression with lemon curd and then on top of that go with light italian meringue flecked with lemon zest (or something equally fluffy and tall). The lemon will be divine with the buttermilk and it will look like you meant for there to be a depression.
-or, you could try Alton Brown’s trick of using ungreased coffee mugs, cooling them upside down on a rack then cutting them out of the mugs.
-or, you could reduce the butter. This will lighten the texture and improve structure. And with such a large amount, I don’t think it will come out dry. If it were me, I would cut it to at least 4 oz.
-If you want to maximize moisture with the reduced butter, oil moisturizes better than butter. But then if you use oil, you may as well switch to Rose’s chiffon recipe, which will shrink to flat (baked 15” ) wrinkled tops.
Good luck and post pics if you can!