In a bowl, add the milk, the oil, the egg whites and the dry yeast powder, mix gently and add the wheat flour, mix very well until a dough forms.
Knead it by hand.
Cover the bowl with plastic wrap and let it rise until doubled in size.
Meanwhile, in another bowl, add the diced potatoes, previously cooked, add the unsalted butter and mash all the ingredients very well until they reach the consistency of mashed potatoes.
Add parsley to taste, the salt, mix.
Get some of this potato mixture with a spoon and round it, making a total of 12 equal sized balls then set aside.
Once the dough has risen, flour your table, place the dough on it and split it into 12 equal parts.
Get each of these pieces of dough and round them into balls.
Now stretch them by hand into circles of about 10cm diameter.
Repeat this process with all the pieces of dough.
To make the filling, place some grated mozzarella on the center of each dough, add the potato balls, close, flip and flatten into a dumpling shape.
Transfer the dumplings to a baking sheet lined with non-stick parchment paper and brush them with 1 egg yolk.
Take to a preheated oven at 190°C (374°F) and bake for 35 minutes.
Serve.