Cover the cubed potatoes with cold salted water in a large saucepan and simmer until tender, about 20 minutes.
While potatoes are cooking, add the butter to a small saucepan and saute the shallot until translucent. Add the buttermilk and warm. Season with salt and pepper.
Once tender, drain potatoes, return to pot, and coarsely mash with a potato masher. Stir in the warmed buttermilk mixture, season with salt and pepper and garnish with chives.