Place the potatoes and salt in a medium saucepan and add enough water to cover. Bring to a boil over high heat. Reduce the heat and simmer until the potatoes are pierced easily with a fork, about 10 minutes. Drain, reserving a few tablespoons of the cooking liquid.
Place the potatoes in a large bowl and partially mash.
Add the warm buttermilk and baking soda, mix well, and mash completely. Stir in the butter and pepper and adjust the seasoning as needed. If you like your mashed potatoes creamy, add the reserved cooking liquid. Serve immediately.