Pre-heat the oven to 350 and get out an 8 x 8 baking dish. The dish should not be greased. Then peel the bananas and mash them thoroughly in a large mixing bowl with a fork. Then add in the flour, sugar, vegetable oil, water, cocoa powder, vinegar, baking soda, cinnamon, vanilla and salt. Stir it all together thoroughly until it becomes a thick batter. Transfer the batter to the baking dish and even it out with a spoon or spatula. Sprinkle the mini chocolate chips on top to form a top crust. Bake the mixture for 35-40 minutes, until a toothpick inserted in the center comes out cleanly. Let the bars completely cool for an hour, then cut them into 16 equal little squares and serve! They keep well in a sealed container for a couple of days too.