Add all the dry ingredients to a large bowl and whisk them together. Take all the wet ingredients and add them to a smaller bowl. Whisk these together too.
Add the wet ingredients to the dry ingredients and whisk them together.
Take half of the batter and put it in another bowl. Add cocoa and additional milk to one half of the batter. Now you have the light and the dark batter.
You can use a round baking tray or the rectangular one. Line it with parchment paper.
There are different ways of creating marble. You can spoon the batter in, adding one spoon of the light batter and one spoon of the dark one, repeating the process until you run out of the batter. This will create a zebra effect as you can see with my cake. You can also pour in a bit of one batter, then a bit of the other, a bit of the first, and a bit of the second. That will also create some swirls. The third way is to pour in one batter, then add the second one and create swirls by using a knife or an iron stick to swirl the batter around inside the tray (don't over-do it though or you might lose the swirls – been there, done that).
Bake at 180°C for 1 hour. Let it cool down a bit before cutting it.