Preheat oven to 180 degrees C and place rack in center of oven. In an ungreased 20 cm square cake pan, stir together the flour, sugar, sifted cocoa powder, baking powder, baking soda, and salt. Add the butter, water, and vanilla extract. With a fork, mix all the ingredients together until well blended. Bake in preheated oven for about 30 minutes, or until the cake springs back when lightly pressed and a toothpick inserted in the center of the cake comes out clean. Remove from oven and place on a wire rack to cool.