Instructions

1. Peel and cut 4 pounds Yukon gold potatoes into rough 2-inch pieces (skip peeling if you will be using a ricer or food mill). Place in a large pot and add enough cold water to cover by about 1 inch. Stir in 2 tablespoons of the kosher salt. Cover and bring to a gentle boil over medium-high heat. Uncover and reduce the heat as needed to maintain a gentle simmer. Cook until knife-tender, 20 to 30 minutes. Meanwhile, heat the oven to 400ºF.
2. Drain the potatoes in a colander. Melt 2 sticks unsalted butter in the now-empty pot over medium heat. Add 4 finely grated garlic cloves, 1/2 cup heavy cream, and the remaining 2 teaspoons kosher salt. Turn the heat off.
3. If using a potato masher, return the potatoes to the pot and mash until your desired consistency. If using a food mill or ricer, work in batches and pass the potatoes into the pot; discard the potato skins.
4. Gently fold the butter and potatoes together with a flexible spatula until the butter is absorbed. Pour in 1 1/2 cups room-temperature buttermilk and gently fold together. It will seem soupy at first, but the potatoes will gradually absorb the liquid and turn into a creamy mixture.
5. Transfer 5 cups of the mashed potatoes into an 8x8-inch or other 2-quart baking dish and spread into an even layer.
6. Stir 2 large egg yolks and 4 ounces shredded aged cheddar cheese if desired into the remaining mashed potatoes. Spread over the mashed potatoes in the baking dish with a flexible spatula into an even layer, going all the way to the edges.
7. Bake until the top is browned in spots and the potatoes are warmed through, 20 to 35 minutes. Garnish with 3 tablespoons finely chopped chives and a pat or two of butter if desired.