Preheat the oven to 350˚ Fahrenheit
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.
Spray or brush a 9x13 inch baking pan with a little oil, and spread batter evenly in pan.
Transfer to oven and bake for 30-35 minutes, until the top is no longer shiny and checked for doneness by poking with a toothpick or cake tester (toothpick should be clean). Let cool for at least 10 minutes before cutting into squares.