Preheat oven to 200°C/180°C fan forced (390°F).
Grease muffin tins or line with patty pans.
Zest and juice the lime.
Cut the feijoas in half and scoop out the flesh. Put into a bowl and lightly mash with a fork.
Top and tail two more feijoas, dice and blitz in the chopping bowl of an immersion/stick blender or food processor. This step is optional but really boosts the feijoa flavor. If you really can't stand the skins you can scoop out the flesh instead.
Add the blitzed whole feijoas to the mash feijoas, along with the lime juice and zest and vanilla and mix well.
In a separate bowl add the the egg and oil and mix well, then add to the mashed feijoas and mix.
In a separate bowl add the flour and brown sugar and mix well.
Add all the wet ingredients to the dry ingredients and mix until just combined. There should not be any dry flour but do not over mix your batter or you will have dense muffins.
Spoon into the lined muffin tins and bake for twenty five minutes or until golden brown.
Remove from the oven and eat straight away while still hot (mind your fingers!) or allow to cool and enjoy later.