Drain the potatoes and leave to cool. For each potato take a slice off across the top of the potato and hollow out the middle with a spoon leaving a space about the size of a dessertspoon. Mash the hollowed out potato with a fork and incorporate a dessertspoon of olive oil.
In the hollow in the potatoes, put some cheese, a little anchovy and then more mozzarella. Close the potato with the mashed potato. Flatten the surface so as to make a lid. Put the potatoes in an ovenproof dish, put a little pat of butter on each and put in a hot oven for 10 minutes.