Preheat the oven to 350F and line an 8×8 baking dish, or spray with non-stick oil.
Add the flour, sugar, cocoa powder, baking soda and salt to the 8×8 dish and stir until completely mixed.
Make two wells in the flour. In one, pour in the oil, and in the second, add the apple cider vinegar and vanilla. Pour the water over top, and then gently stir until a cake batter is formed.
Bake for approximately 28 minutes, until a toothpick poked in the center comes out clean. Allow to cool completely before coating with icing. Store in a sealed container on the counter for up to 3 days.
