Nat Mur craves bitter things, that's why they like dark chocolate, as gien writes.
They do also like milk chocolate because it is sweet and creamy like breastmilk.
In a case where chocolate craving is prominent it is useful to consider what aspect of the chocolate the patient craves - eg. the sweet, milkiness, the bitterness, the bitter-sweet. That can guide you to a prescription through understanding the case. It's the kind of practical thinking that makes for a good homoeopath, rather than routine rubric use.