Preheat the oven to 180C/350F/gas mark 4. Grease an approx 11.5 x 21.5 cm / 4.5 x 8.5 in loaf tin and line it with a strip of baking parchment; leave some overhanging on either side.
Place the bananas in a blender along with the sunflower oil, brown sugar, maple syrup or agave, vanilla extract and non dairy milk and blend until the mixture is very smooth.
In a separate bowl, sift together the plain flour, cocoa powder, baking powder, bicarbonate of soda and salt.
Pour the wet ingredients into the dry and fold in until no pockets of flour remain; make sure that you don't over-beat the batter though as this can make the banana bread tough.
Fold in the chocolate chips then scrape the batter into the prepared tin. Spread it level and bake for about 45-55 minutes.
It is very, very important that you do not bake this vegan chocolate banana bread until a skewer comes out clean. You want the skewer to come out a bit gunky - not wet batter, but definitely with a bit of thick, sticky batter on it. If you leave it in the oven until the skewer comes out clean then it may well end up being a bit dry so do keep a close eye on it towards the end of cooking.
Once the banana bread is done, remove it from the oven and use the overhanging strip of baking parchment to lift it out of the tin on to a wire rack (make sure that you remove the baking parchment too). Leave to cool.