Start by putting the plain flour in a bowl with the baking powder, salt and caster sugar and whisking through.
Put the banana in another bowl and mash to a smooth paste.
Now add the egg, vanilla and oil.
Add all of this to the dry mix with the milk and whisk till you have a really thick, spoonable batter.
Set aside and start heating the oil in a medium pan. You need enough oil to be able to deep-fry these little balls.
As soon as the oil gets to 150°C, drop the batter in 1 teaspoon at a time, using another teaspoon to tease the batter off the spoon.
Add enough so that you can still comfortably fry the balls and they can move around in the oil.
They should only take 4–5 minutes to cook.
Drain onto some kitchen paper and roll in caster sugar.
Do this to all of the batter and enjoy the doughnuts while they are still warm.
TIP: You can dust with icing sugar instead of caster sugar if you are after a powdered doughnut rather than a sugared doughnut.
