Heat the oven to 160° Celsius with top and bottom heating but no fan.
Break the eggs and add the sugar, mix until the mixture becomes creamy.
Add the milk and oil and mix to blend.
Also add the vanilla extract.
Add the baking powder and the flour gradually and mix until the mixture is smooth again.
Pour half of the mixture in an another pot.
To the remaining mixture add two tablespoons of cacao powder and mix to blend.
Grease the bottom and the sides of the baking dish/tin.
Start pouring the mixtures one by one starting from the middle. Pour some of the cacao mixture first and then some of the vanilla mixture. Repeat until both are finished. By the end the mixtures are finished all the baking dish/tin will be covered, as with each mixture addition the previous addition will spread out to the sides.
With a tooth pick make slicing traces from the center of the cake to the sides. Then repeat making new traces from the middle of the slices, this time from the sides to the center.
Bake for about 40-50 minutes. Check by poking a knife in the middle of the cake, if the knife comes out clean it shows that the cake is ready.
Enjoy.