Julie: Okay, I will try one with 15% starch and less baking powder. And I will keep a better eye on the cake near the end of the baking time. Too bad both types of American flours are bleached. I will try other stores with American products. Maybe some of them do have bleached flour.
Freshkid: In the Netherlands we can get fine sugar and in Belgium I found ultrafine sugar. The latter is actually meant for decorating and cold recipes. Here is a (crude) translation of the information about the Belgian sugar:
I think putting this sugar in a food processor would not make a difference.