Sift the flour together with the baking powder, and mix well with a whisk. Add in the sugar and cocoa powder and mix well.
Pour the oil+water mix into the dry ingredients, and gently mix with a whisk until combined and lump-free.
Spray cake pan with loose base. Pour in the cake batter and even the top with spatula.
Bake in the middle of the oven for about 40 minutes, then insert a toothpick in the middle of the cake. If it comes out dry, the cake is done, otherwise continue to bake for a further 5 minutes, then check again.
Take the cake out from the oven and allow it to cool completely in the pan.