Combine all of the dry ingredients (flour, sugar, baking soda, cocoa, salt) in a bowl. Stir until well mixed.
Add the vinegar, oil, vanilla and water. Stir with a whisk, you don't need a mixer. Lay out a 9″ (23 cm) x 13″ (33 cm) oven dish with baking paper.
Pour the mixture in it and bake at 356°F (180 °C) for 40-45 min. Take it out of the oven and leave it to cool.
Optionally, sprinkle with powdered sugar or pour a chocolate glaze over it.