There are two easy things you can do to ensure that the end product is moist.  First of all, do not over mix the ingredients.  A great way of achieving this is to mix the dry and wet ingredients separately and then add them together, mixing until just combined. It is also important to not over bake the Nutella swirl muffins.  We want them to be baked through and golden brown but not overdone.  Cooking them for too long will create a dry end product that is not moist.