These Polish dumplings are traditionally served with soft, browned onions or sour cream.
Place potato in a large pan, and cover by 2 inches with cold water. Add 1 tablespoon salt, and bring to a boil. Lower heat to medium, and cook until tender, about 40 minutes. Drain, and holding the potato with a kitchen towel, peel while hot. Pass potato through a food mill, fitted with the finest disk, or a potato ricer into a large bowl.
Add goat cheese, shallot, egg yolk, cream, sage, 1 teaspoon salt, and pepper, and mix well using a wooden spoon.
To form dumplings, place a circle of pierogi dough on a work surface, and put a heaping teaspoon filling toward the front of the circle. Moisten the edges of the dough with water, and fold in half around the filling to form a semicircle, firmly pinching the edges closed. Set aside on a lightly floured baking sheet. Continue to fill and form pierogi, keeping them covered with plastic wrap, until all the dough is used up.
Meanwhile, bring a large pot of salted water to a boil, and add half the pierogi. Boil gently until they float to the surface, about 2 minutes, and cook for 3 to 4 minutes more. Lift out with a slotted spoon, and repeat with remaining pierogi. Serve immediately.
Save time by cooking and ricing the potato for this filling together with the one for the pierogi dough.