Line a small loaf pan all sides up.

Beat butter and sugar till light and fluffy with an electric mixer. Gradually add beaten eggs (100g egg + 20g egg yolk) into it, followed by vanilla extract and mix well.

Sieve in plain flour and baking powder, fold well with a rubber spatula. Take out 1/3 of the of the batter into a bowl and fold well with the cocoa powder and milk.

Layer the plain and chocolate batter (3 each) alternately into prepared pan or lightly swirl the batters with a fork to create marbling effects.

Bake in preheated oven at 180 deg C for 15 minutes and lower temperature to 160 deg C for another 20-25 minutes. Leave cake in pan till completely cool before slicing.