Preheat the oven to 180 C. In a large bowl, stir together the flour, sugar, cocoa powder, baking powder and salt.

Pour in water, vegetable oil and vanilla; mix until well blended. Spread evenly in a 23x33cm (9x13 in) baking tin.

Bake for 25 to 30 minutes in the preheated oven, until the top is no longer shiny. Let cool for at least 10 minutes before cutting into squares.