1. In a large mixing bowl, combine eggs and sugar. Using a hand-held electric mixer or wire whisk beat until the mixture is creamy and light in color.
2. Add milk and oil, and continue beating until well blended.
3. In a separate bowl, combine and mix flour, vanilla powder and baking powder. Gradually add the flour mixture to the wet ingredients and beat just until the batter is smooth and the dry ingredients are thoroughly incorporated. (DO NOT OVERBEAT to prevent air pockets from forming in the batter. The batter should be of medium consistency, not too thick and not too watery.
4. Divide the mixture into 2 equal portions. Keep one portion plain. Add cocoa powder into another and mix well.
5. Preheat the oven to 350F (180C).
6. Lightly grease a 9" round pan.
7. The most important part is assembling the cake batter in a baking pan. This is what you do. Scoop 3 heaped tablespoons of plain batter (you can also use a ladle that would hold 3 tablespoons) into the middle of the baking pan. Then scoop 3 tablespoons of cocoa batter and pour it in the center on top of the plain batter. IMPORTANT! Do not stop and wait until the previous batter spreads - KEEP GOING! Do not spread the batter or tilt the pan to distribute the mixture. It will spread by itself and fill the pan gradually. Continue alternating the batters until you finish them.
8. Bake in the oven for about 40 minutes. Do not open the oven door at least the first 20 minutes or the cake will shrink and will not rise. To check if the cake is ready, insert a toothpick into the center. It should come out clean when ready. Remove from the oven. Immediately run a small thin knife around the inside of the pan to loosen the cake, and then invert the cake onto a cooking rack. Turn the cake back over and let cool. You can sprinkle the top of the cake with some powdered (confectioner’s) sugar or leave it plain.