Beat the eggs, add sugar and stir until dissolved. Then add the oil, milk and the flour with the baking soda and vanilla.
Sift the flour so it gets airy. Separate a little of the mixture and mix it with the cocoa. Add it back to the mix, which you've put in the baking dish, in a zigzag pattern.
Bake at 356°F (180 °C) in a preheated oven, until ready. Check to see if it is ready with a toothpick.