Pre-heat the oven to 180°Celsius°C. Line a baking tray with parchment paper.
Puree the bananas in a food processor.
Add the prunes, oats, flax seeds, cinnamon and the oil. Whiz for a few seconds. Add gradually the milk, pulse again for a few seconds then add more milk until you get a thick, textured batter.
Form six or eight balls from the mixture, then flatten them slightly between your palms. Put the cookies on the baking tray and bake in the oven for about 20 minutes until golden.
Transfer the cookies to a wire-rack and let them cool.
Keep them in an airtight jar or tin.