Peel and cut 5 pounds russet 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 2 cups half-and-half and the remaining 2 teaspoons kosher salt in a small saucepan over low heat, stirring to dissolve the salt, until just starting to simmer around the edges. Turn the heat off and cover to keep warm.

When the potatoes are ready, drain them in a colander. Melt 2 sticks unsalted butter in the now-empty pot over medium heat. Turn the heat off.

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 of butter; discard the potato skins.

Gently fold the butter and potatoes together with a flexible spatula until the butter is absorbed. Pour in the half-and-half and gently fold together. It will seem soupy at first, but the potatoes will gradually absorb the liquid and turn into a creamy mixture.

Taste and season with more kosher salt as needed. Transfer to a serving. Garnish with finely chopped chives, freshly ground black pepper, and a pat or two of butter if desired.