To achieve the proper texture, it is important to cook the potatoes thoroughly; they are done if they break apart when a knife is inserted and gently wiggled. Buttermilk substitutes like clabbered milk do not produce sufficiently tangy potatoes. To reduce the likelihood of curdling, the buttermilk must be brought to room temperature and mixed with cooled melted butter.
2 pounds Yukon Gold potatoes, peeled and cut into 1-inch chunks
6 tablespoons (¾ stick) unsalted butter, melted and cooled
2/3 cup buttermilk at room temperature
Ground black pepper
1. Place the potatoes in a large saucepan and cover with 1 inch cold water; add 1 tablespoon salt. Bring to a boil over high heat, then reduce the heat to medium and simmer until the potatoes break apart very easily when a paring knife is inserted, about 18 minutes. Drain the potatoes briefly, then immediately return them to the saucepan set on the still hot burner.
2. Using a potato masher, mash the potatoes until a few small lumps remain. Gently mix the melted butter and buttermilk in a small bowl until combined. Add the butter--buttermilk mixture to the potatoes; using a rubber spatula, fold gently until just incorporated. Adjust the seasonings with salt and pepper to taste; serve immediately.