Marble Butter Cake

Havent been baking marble cakes for a long time cos I can't seem to create a nice marble effect. Sigh .... looks like I failed again ....
250g softened butter
210g plain flour
200g castor sugar
5 eggs
1/2 tsp baking powder
20g cocoa powder
2 tbsp milk
1. Cream butter and sugar until light and fluffly.
2. Add eggs, one at a time until well incorporated before adding the next egg.
3. Add milk.
4. Fold in sieved flour and baking powder. Mix well.
5. Remove 1/3 batter and add cocoa powder to it.
6. Pour remaining 2/3 batter into cake tin then follow by chocolate batter.
7. Use a fork to lightly create marble effects and bake at preheated 180 deg for about 50 minutes.