Start by adding into a bowl the wheat flour, the milk, the unsalted butter (you can use margarine if you prefer) and the salt.
Mix it all together very well with a spatula and when you can no longer mix with it, use your hands until it forms a smooth and uniform dough.
Once you’ve done that, cover with a plastic bag touching the dough and let it rest for 1 hour in the fridge.
Now, into a large bowl, add the peeled and cooked potato, mash it very well.
Add the mayonnaise and mix well into a mash. Set aside.
After the indicated time, take the dough out of the fridge and put it on the table. Stretch it into a circular shape, until it reaches about 22cm in diameter.
Add the mozzarella cheese to the center of the dough and the mash we’ve just made, spreading well.
Finally, add another 50g of shredded mozzarella cheese all over the mash.
Now, close the bread, pulling the edges together over the filling to make sure the dough is well sealed.
Now get the rolling pin and lightly roll it over the bread dough to stretch it out and spread the filling.
Into a skillet over very low heat, brush some butter all over its bottom, place the bread and let it fry until golden brown and crusty.
While the bottom part is frying, brush some melted butter all over the upper side, flip the bread and fry the other side as well, over very low heat.
Serve.