I found a great Feijoa chocolate cake recipe at Thinking About Food. I'm not a confident cake maker, they are a lot bigger than mini muffins and require a bit more concentration when trying to bake perfectly.
I had some old Dunedin friends - Tom and Claire as official cake samplers and they enjoyed it. Wahoo! I've just pulled a Feijoa cobbler out of the oven and now, the bag of feijoas finished.