Slightly adapted from Pragerway, this recipe makes one 8x8-inch cake, 12 servings.
1 cup granulated sugar
1-1/2 cups unbleached all-purpose flour
1/3 cup unsweetened cocoa powder (I used Dutch processed, because that's what I had)
1 tsp baking soda
1/2 tsp table salt
2 tsp pure vanilla extract
1/2 cup canola oil
2 Tbsp distilled white vinegar
1-2 tsp confectioners sugar
Preheat oven to 375°F, with a rack set at the upper middle level. Make sure the oven is fully preheated before you begin to mix the ingredients, as you have to work quickly once you get started.
Spray an 8x8-inch baking pan with baking spray.
In a large mixing bowl, whisk together the sugar, flour, cocoa powder, baking soda, salt, vanilla extract, oil, and 1 cup of cold water. Blend well. The batter will seem a bit thick.
Then, add the vinegar. Stir quickly! Pour the batter into the prepared baking pan, and put it into the oven immediately. Don't dawdle; a chemical reaction happens with the vinegar and baking soda, and it needs heat to work its magic. Get the pan into the oven as fast as you can.
Bake for 20-25 minutes, until the cake is slightly puffed in the middle. Remove the pan to a cooling rack, and let sit for at least 5-10 minutes. Use a fine strainer to sift confectioners sugar over the top. Serve as is, or with a dollop of whipped cream or vanilla frozen yogurt.