Preheat the oven to 350 degrees. Grease and flour square pan, 8"x8"x2". In a small mixing bowl, combine the flour, sugar, cocoa, baking soda and salt. Add water, oil, vinegar and vanilla. Stir in mashed ripe banana; beat until smooth. Pour into the prepared pan. Sprinkle chocolate chips over top. Bake 35 to 40 minutes or until a wooden toothpick inserted in center comes out clean. Cool in the pan on a wire rack. Cut into squares. Serves 6.