When passive voice is used, the focus of the sentence is removed from the subject that is performing the action and instead focuses on the object or person that the action is being done on.
In the above, Darani was the subject engaged in the action of milking so the correct passive voice would be one where the cow is now the subject. The correct word to use is "being" not "been" and the tense has to be in the present because the action is a present continuous one.
