Begin by setting your oven to 350°F (175°C) to preheat.

In an 8x8-inch ungreased cake pan, combine the flour, sugar, cocoa powder, baking soda, and salt by sifting them together. Create three small wells in the dry mixture. In one well, add the vegetable oil; in the second, pour the cider vinegar; and in the third, add the vanilla extract.

Pour the cup of water over all the ingredients in the pan, then mix everything together with a fork until the batter is smooth and well combined.

Place the pan in the preheated oven and bake for 30 to 40 minutes. The cake is done when a toothpick inserted in the center comes out clean.