5medium Yukon Gold potatoespeeled and cut into quarters
1large sweet potatopeeled and cut into 1 inch chunks
5tablespoonsunsalted buttercut into individual tablespoon-sized slices, divided
3medium sized leekswhite and light green parts only, sliced and well rinsed
1teaspoonkosher or coarse saltplus more to season
Freshly ground black pepperto taste
1 ½ to 2cupswhole milk or half and halfwarmed
Place the Yukons and the sweet potatoes in a large pot with a lid, cover by at least 2 inches with cold water and place over high heat. Cover the pot and bring to a boil. Uncover the pot, lower the heat so that the water stays at a simmer, and stir in the teaspoon of salt. Simmer uncovered for about 20 minutes, until the potatoes are very tender.
While the potatoes are cooking, heat about two tablespoons of butter in a large skillet over medium heat. Add the leeks and cook, stirring frequently, until they are very tender and lightly browned, about 20 minutes. Make sure the heat isn't too high – you want them to slow cook and very lightly brown. Season lightly with salt and pepper.
Drain the potatoes. Pass them through a ricer or a food mill, and transfer the mashed vegetables back to the pot, or alternately you can mash them in the pot with a potato masher (this method will yield a few lumps, which some people like).
Add the remaining butter and 1½ cups of the milk to the pot and stir or whisk over high heat until hot, creamy, and smooth. Add the remaining milk as desired to get to the consistency you like. Season with additional salt as needed, and the pepper.
Transfer the mashed potatoes to a serving bowl and distribute the sautéed leeks on top. Serve hot.