Place the potatoes in a large pot of cool water. (All the potatoes should be fully submerged.) Season the water very generously with salt and boil the potatoes until tender, about 30 minutes. Drain the potatoes and discard the water.
Place an oven-safe serving dish in the oven to warm.
Press the potatoes through a ricer. (If too large, cut in half before pressing.)
Meanwhile, heat the milk in a small saucepot over medium heat.
Pour the warmed milk over the riced potatoes. Fold in the butter to combine. Season with salt if necessary. Transfer to the warmed serving dish and serve.