Into a bowl, add the sugar and the eggs, beat them well.
Add the oil, the wheat flour, the milk, and beat well again.
Finally, add the baking powder, mix gently.
Then equally split the batter in 2 bowls.
To one of the bowls, add the cocoa powder and mix until combined.
To the other bowl, add the vanilla sugar and mix.
Now, grease a cake tin with the unsalted butter, spreading it out very well.
After doing that, pour both batters in the cake tin at the same time, one on each side.
Using a barbecue skewer, make spiral movements in the batters, mixing them gently.
Take to a preheated oven at 180°C (356°F) and bake for 45 minutes. Baking time may vary depending on the power of your oven.
Finally, unmold and sprinkle icing sugar all over the cake.
Serve.