Combine the potatoes and 1 teaspoon of the salt in a medium pot with water to cover by 1-inch. Bring to the boil. Reduce the heat to medium-low and simmer uncovered until the potatoes are fork tender, about 15 minutes. Drain in a colander.
Return the potatoes to the cooking pot and place over medium-low heat. Add the buttermilk, butter, remaining 1/4 teaspoon salt, and black pepper and mash with a potato masher or heavy fork until fluffy, about 4 minutes. Remove from the heat and adjust seasoning, to taste. Serve immediately.
Yield: 4 cups