Grease and line a 1kg loaf tin.
Preheat the oven to 160°C.
Mash 3 of the bananas into a measuring jug with the vegetable oil and vanilla.
In a mixing bowl, combine the dry ingredients.
Add the wet mix to the dry mix and combine.
Pour into the lined loaf tin and top with the 4th banana, halved lengthways.
Bake for 40-45 minutes until golden on top and springy to touch in the centre.