Preheat your oven to 350°F (175°C).
In an 8×8 inch ungreased baking pan, add the flour, sugar, cocoa powder, baking soda, and salt. Mix them around right in the pan until they look evenly combined.
Make three small wells in the dry ingredients. Pour the vinegar in one, vanilla in another, and oil in the third.
Pour the water over everything and stir with a fork or whisk until no streaks remain. Don’t overthink it, it’ll all come together.
Pop the pan into the oven and bake for 30–35 minutes, or until a toothpick poked in the center comes out clean.
Let it cool right in the pan. Serve as is, or dress it up with powdered sugar, frosting, or even a scoop of vanilla ice cream.