To make the pierogi filling (Up to 1 day in advance):

1. Melt the butter in a skillet set over medium-heat. Add the leeks to the skillet and cook, stirring occasionally, for approximately 5 minutes, until tender.
2. Put the potatoes into a pot and add just enough cold water to cover. Bring to a boil and cook until the potatoes are soft, approximately 20 minutes.
3. Drain the potatoes in a colander and press out any remaining moisture. Add the cooked leeks, cheese and milk to the potatoes and mash together, until smooth (You can also do this in a stand mixer, with the paddle attachment).
4. Refrigerate the mixture until cold, for approximately one hour or up to one day in advance (The filling is much easier to work with when chilled).

To make the dough:

1. In a large bowl or stand mixer (with the paddle attachment), mix the flour and salt. Add the eggs and sour cream and combine. The dough should feel slightly tacky and have an elastic consistency to it.
2. Turn the dough out onto a lightly floured surface, and pat it into a disc. Cover the dough in plastic wrap and refrigerate for at least one hour.
3. Once the dough has chilled, roll it out into a thin circle, approximately ? inch thick – This will take some effort given its elasticity.
4. Using a standard biscuit cutter, cut the dough into rounds. Continue rolling and cutting until all of the dough is used. You will have approximately 20-25 rounds.
5. In the palm of your hand, take one round at a time, and fill the center with a generous tablespoon of the potato mixture. Gently fold the round in half, pulling the edges away and pinching to enclose the filling. Work from one end to the other, ensuring that the entire dumpling is sealed.
6. As you work, set the filled pierogi aside on a lightly floured surface.
7. Bring a large pot of water to a boil. Working in batches, drop no more than 6 pierogi at a time into the water. You will know they are done when they float to the surface. Remove with a slotted spoon and place on a towel to drain and cool.
8. The pierogi are best served immediately, with melted butter. Alternately, pan-fry the pierogi in a skillet with butter or olive oil.

To make the frizzled leeks:

1. Cut the leek in half crosswise, and then cut each piece in half lengthwise, so that you have four long sections. Cut each section into thin strips.
2. Heat the oil in a large pot set over medium-high heat. Once the oil is very hot, drop the leeks into it, and fry for 1-2 minutes, until lightly browned and crisp. Remove the leeks from the pot using a slotted spoon, and place on a paper towel-lined plate to absorb any excess oil. To serve, sprinkle the frizzled leeks over the top of the pierogi.