Preheat oven to 350°. In a bowl, combine flour and baking soda. In another bowl, whisk brown sugar, water, banana, oil and vanilla. Stir into dry ingredients just until moistened. Transfer to a greased 8-in. square baking pan., Bake until a toothpick inserted in the center comes out clean, 30-35 minutes. Cool on a wire rack. Dust with confectioners' sugar.