Preheat oven 350 F (180 C).
Line a 9x9 inch square baking dish with parchment paper.
In a bowl combine the flour, sugar, salt and baking powder. Whisk to combine. Add the water, oil and vanilla. Beat until just incorporated. Add citrus zest if using any and mix to combine.
Bake at 350 (180 C) for 45 minutes or in a fan oven at 160 for 40-45 minutes, until toothpick inserted comes out clean.
Transfer to a cooling rack and cool completely.
Cut into squares.
A round 9 inch pan will work in this recipe as well.
