Preheat your oven to 350°F (175°C).
In an ungreased 8×8 baking dish, whisk together flour, sugar, cocoa, baking soda, and salt.
Make three small wells in the dry ingredients. Pour the vinegar in one, vanilla in the second, and oil in the third.
Pour the water over everything and mix well with a fork until fully combined.
Bake for 30–35 minutes, or until a toothpick comes out clean.
Let cool in the pan, slice into squares, and try not to eat it all in one sitting.