Whisk 1 cup of warm water, ¼ cup of whole milk, 2 tablespoons of sour cream, 1 large egg, 3 tablespoons of extra-light olive oil, and 1½ teaspoons of fine sea salt.
With the dough hook of a stand mixer, mix in 4 cups of all-purpose flour, half at a time. Knead for 2-3 minutes, until smooth and elastic. Cover and rest for 30 minutes.
Meanwhile, boil 900g of russet potatoes, peeled, for about 25 minutes, until a fork pierces them easily. Drain, cool for 5 minutes, and mash until smooth.
Mash in ½ teaspoon of salt, 2 tablespoons of melted unsalted butter, and 55g of softened cream cheese, then ¾ cup of shredded mozzarella. Partly cover and set aside.
Roll out the dough thinly on a floured surface and cut into rounds. Put a half-tablespoon of filling on each, fold in half, and pinch the edges to seal.
Cook in batches in a large pot of boiling salted water for about 2 minutes, until they float. Lift out with a slotted spoon.
