1Adjust oven rack to middle position and heat oven to 350 degrees. Coat 8 inch square baking dish with nonstick cooking spray.
2Whisk flour, sugar, cocoa, baking soda and salt together in pan. Make one large and 2 small craters in the dry ingredients. Add oil into the large crater and and vinegar and vanilla into remaining small craters.
3Pour water into pan and mix just until a few streaks of flour remain. Immediately put the pan in oven.
4Bake until toothpick inserted in center of cake comes out with a few moist crumbs attached, about 30 minutes. Cool in pan, then dust with confectioners sugar. If tightly wrapped, cake will keep for 3 days at room temperature.