I did them in two batches of three when I made mine, and that seemed to work ok (do the same thing with the lemon and fruit bar too - I cut them into eight bits 25 seconds is usually enough for each four biscuits to leave them a bit soft and chewy instead of brittle)
Keep on trying - I've lost count of the number of shakes I must have wasted trying to make muffins! Hope the burnt bits aren't too bad!