In a separate bowl, whisk the egg yolks with the sugar until fluffy and creamy.
Stir in the oil and water. Then add the flour and baking powder.
Gently fold in the egg whites.
Divide the dough into two parts and mix one half with the cocoa powder.
Put one or two spoons of white dough into the middle of your baking tin. It will slowly flow in all directions. Then put the same amount of dark dough directly onto the first, not next to each other.