In a bowl, place the medium boiled potato, add the milk and mash the potato with a fork, until fully combined with the milk.
Add the dry yeast powder, the salt and the sugar, mix very well again. Let this mixture rest for 5 minutes and add the butter, the egg and the wheat flour, mix very well until combined. Add parsley and chives to taste and mix. Cover the bowl with a cloth and let the dough rise until it doubles in size.
Meanwhile, evenly grease a baking dish with butter.
Transfer the dough to the baking dish and spread it out evenly. With the spatula, split the dough into 3 equal pieces, cover with a cloth and let it rest for 15 to 20 minutes.
Fill the cuts we made with the spatula with slices of mozzarella cheese, but that's optional and to taste.
Brush an egg yolk over the dough.
Bake in a preheated oven at 200°C (392°F) for 40 to 50 minutes.
Do the toothpick test.
Serve.