1. Adjust oven rack to middle position and heat oven to 350 degrees. Coat 8-inch-square baking pan with nonstick cooking spray.
2. Whisk flour, sugar, cocoa, baking soda, and salt together in pan. As shown in photo 1, make 1 large and 2 small craters in dry ingredients. Add oil to large crater and vinegar and vanilla separately to remaining small craters. Pour water into pan (photo 2), and mix until just a few streaks of flour remain (photo 3). Immediately put pan in oven.
3. 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. (If tightly wrapped, cake will keep for 3 days at room temperature.)
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