Preheat oven to 180 degrees C. Grease and flour a 2 litre ring-shaped tin or similar.
In a large bowl combine eggs, oil, caster sugar and vanilla sugar. Beat on high speed for about 5 minutes. Beat in flour. Reserve approximately 1/4 cup of batter and set aside.
Pour remaining batter into prepared tin. Mix cocoa into reserved batter. Drop by spoonfuls onto the batter in the tin. Using a skewer or the tip of a knife to swirl the two batters to achieve a marbled effect.
Bake in the preheated oven for 30 to 40 minutes or until a toothpick inserted into the centre of the cake comes out clean. Allow to cool.