In a bowl mix butter with sugar and whisk well until they are well mixed.

Add eggs one by one while mixing well.

Add milk and vanilla extract and continue mixing well.

Add flour in small portions while mixing until they are well incorporated.

Divide the mixture into two equal portions.In one portion add cocoa and mix well.

Grease baking tin with butter and dust using flour.

Pour your mixture in alternations.Pour small amount of plain mixture,then add the cocoa mixture, continue adding all the mixture in that manner.

Preheat the oven for five minutes at 180 degrees and bake for 30minutes.