Pre heat the oven to 180 degrees Celsius. Grease and line an 8 inch round cake tin.
Beat the eggs and sugar together till they are light and fluffy.
Add the oil and mix well. Add milk (two tablespoons of milk for chocolate batter) and vanilla extract. Again mix together till it is well blended.
Sieve together the all purpose flour, baking powder and salt.
Add the dry and wet ingredients together, and mix it well till the batter is smooth.
Divide the batter into two parts. In one portion, add the cocoa powder dissolved in two tablespoons of milk. Keep the other portion plain.
For the most important step. Use two separate ladles for the two batters. Start by pouring a ladleful of the plain batter in the middle of the cake tin. Then pour a ladleful of the chocolate batter in the centre on the top of the plain batter. Repeat this process till the batters
Do not tilt the pan. The batter will spread and fill the pan.
Bake for 40 minutes or till a skewer inserted in the centre comes out
Cool and slice even pieces to serve.