Take the buttermilk out of the fridge about 30 minutes before you start cooking so it loses its chill. Shake the carton well before measuring.
1 cup low-fat buttermilk
Peel the potatoes and cut them into chunks about 1 ½ inches wide. Drop them directly into a large pot with cold water to keep them from discoloring while working through all the potatoes. Try to keep the pieces the same size so they finish cooking at the same time. When done cutting, be sure cold water covers the potatoes by 1 inch. Stir in 2 teaspoons of salt.
3 pounds Yukon Gold potatoes, 2 teaspoons salt, Cold water
Set the pot over high heat and bring to a boil, about 10 to 12 minutes. Reduce the heat to medium-low so the water bubbles gently. Cook uncovered until a fork slides into a chunk with no resistance, about 8 to 10 minutes. Do not let them go much past this point, because overcooked potatoes soak up water and fall apart.
While the potatoes cook, pour the buttermilk into a small saucepan and add the butter and olive oil. Set over low heat and stir often until the butter melts and the mixture is just warm to the touch, about 1 to 2 minutes. Do not let it simmer or boil. Remove from the heat.
1 tablespoon unsalted butter, 1 tablespoon olive oil
Scoop out about ½ cup of the potato cooking water with a measuring cup and set it aside. Drain the potatoes in a colander.
Return the drained potatoes to the empty pot and set it over low heat. Let them steam dry for 1 to 2 minutes, gently shaking the pot once or twice, until the steam slows down and the potatoes look dry on the outside. Turn off the heat.
Mash the potatoes with a hand masher using firm up and down presses, about 1 to 2 minutes, until mostly smooth. Do not stir, whip, or beat them.
Pour in ¾ of the warm buttermilk mixture and gently fold it in with a rubber spatula or wooden spoon using about 6 to 8 folds. Add the rest of the buttermilk mixture, salt, and pepper and fold again. If the potatoes feel too dry, stir in the reserved cooking water a couple of tablespoons at a time until the potatoes are spoonable. You may not need any reserved water.
Taste and add more salt or pepper if needed. Stop mixing as soon as everything is combined.
¾ teaspoon salt, ½ teaspoon freshly cracked black pepper
Spoon the potatoes into a warm serving bowl and top with the chives.
2 tablespoons thinly sliced fresh chives