This was once called "Depression Cake", due to the economic times, and it may see a revival yet again. Cooks Illustrated has renamed it "Wacky Cake".
Do not mix this in a bowl first, mix it right in the cake pan.
Adjust oven rack to middle position and heat oven to 350°F Coat 8-inch-square baking pan with nonstick cooking spray.
Whisk flour, sugar, cocoa, baking soda, and salt together in pan. Make one large and two small craters in dry ingredients. Add oil to large crater and vinegar and vanilla separately to remaining small craters. Pour water into pan and mix until just a few streaks of flour remain. Immediately put pan in oven.
Bake 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.