Pre-heat the oven to 175°C (350℉).
Grease and line a loaf pan - mine is 26cm x 13cm (10" x 5").
In a large bowl, mash the bananas well.
Add the oil, maple syrup, eggs, vanilla and milk, and whisk to combine.
Sift together the flour, baking powder, baking soda, and salt then whisk just until everything comes together.
Divide the mixture into two.
In one bowl, add the cocoa and the other add the cinnamon (see note 2). Mix them until everything is incorporated.
Pour the half the cinnamon into the prepared pan.
Pour the cocoa batter over the top, and swirl it through.
Bake for around 45-50 minutes until and inserted skewer comes out clean.
Remove from the oven, allow to cool in the pan for around ten minutes, then turn it out onto a wire rack. Allow to cool. The cake is lovely warm, but just as good cold. I find the flavour intensifies over time. The cake also freezes well.