Line a small loaf pan all sides up.
Beat Butter (150 gram) and Caster Sugar (85 gram) till light and fluffy with an electric mixer. Gradually add the Small Egg (2) and Egg (1) Yolk into it, followed by Vanilla Extract (1 teaspoon) and mix well.
Sieve in All-Purpose Flour (120 gram) and Baking Powder (1 teaspoon). Fold well with a rubber spatula. Take out a third of the batter into a bowl and fold well with the Unsweetened Cocoa Powder (15 gram) and Milk (25 milliliter).
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 degrees C (350 degrees F) for 15 minutes.
Lower temperature to 160 degrees C (325 degrees F) for another 20-25 minutes. Leave cake in pan until completely cool before slicing. Enjoy!