1. Preheat oven to 350 degrees. In a large mixing bowl, whisk together flour, granulated sugar, baking powder, and salt; set aside. In a medium bowl, mash banana. Add oil, egg yolks, vanilla, and 3/4 cup cold water; mix to combine. Add to flour mixture, and whisk batter until smooth.

2. In a large bowl, place egg whites and cream of tartar; using an electric mixer, beat until stiff peaks form. With a rubber spatula, gently fold into batter just until combined (do not overmix). Pour into a 10-inch angel-food cake pan with legs. Bake until a toothpick inserted into center of cake comes out clean, about 55 minutes. Invert pan onto legs over counter; let cool completely. Reinvert pan, and run a thin metal spatula around edge to release onto platter. Dust with confectioners' sugar. Serve.