Originally Posted by epicurian:
“Woe is me. I wonder if not enough air was the reason. I was worried about working the mixture too much for fear of over-developing the gluten in the flour and ending up with a chewy cake. I really don't get this baking thing.”
That's why many (most?) sponge cake recipes have you fold in the flour gently at the end. In that way you avoid overworking the gluten or knocking out the air that you have lovingly incorporated. My mother always just bunts all of her ingredients in the foodprocessor at the same time and blends to a batter, whereas I use the electric mixer to cream the butter and sugar, then mix in the eggs, then fold in the flour and other dry ingredients using a metal spoon and a gentle touch.
Consequently, my cakes rise better and are lighter than my mum's!
in this case, though, I do agree that your sinkage sounds like cooking too quickly at too high a temperature.