Preheat the oven to 180 degrees.

Cream the sugar and butter with a mixer till white and fluffy.

Add the eggs and vanilla and mix till it is well blended.

Fold in flour (use a wooden spoon and gently add in).

Add in some milk so that the batter has dropping consistency.

Take 1/3 of the batter and put into the bowl where your chocolate mix well.(but don`t over mix)

Grease the cake tin and layer the bottom part with yellow batter, then, drop the chocolate and yellow batter alternately. After that twirl with a chopstick.

Bake at 180 degree Celsius for until a toothpick inserted on the top of the cake comes out smoothly.