Peel potatoes, cut them into large pieces (about 2-inch cubes), and place them in a large pot with heavily salted water and bring to a slow boil over medium-high heat.
While potatoes are cooking, cut butter into small cubes and set aside.
When potatoes are fork tender, drain well and return to pot; let sit for 5 minutes to dry out. Transfer potatoes to the bowl of a stand mixer fitted with a paddle attachment. With the mixer on low, add butter and cream until potatoes are smooth. Stir in scallions, if using, and season generously with salt and freshly ground black pepper.