I also wrote 'Quel gâteau est-ce qui tu commandes' and had it marked wrong. I think it is correct. In spoken French est-ce-que is contracted so much it sounds like one syllable so it doesn't sound wordy.
I tried "Quel gâteau est-ce que tu commandes ?" I thought that est-ce que was acceptable for one-syllable interrogative adjectives but not for two-syllable ones. My response might be too wordy, but is it technically correct?