Put the potatoes in a medium saucepan, cover with cold water, and season with salt. Bring to a simmer over medium heat and cook until potatoes are very tender, about 20 minutes.

Heat the milk and butter in a small small saucepan until hot. With a potato masher or large fork, roughly smash the potatoes, while adding the hot liquid. Stir in half of the scallions, season with the salt and pepper, to taste. Transfer to a serving bowl and scatter the remaining scallions on top before serving.