Add potatoes to a pot of cold, salted water. Cook until tender (about 15-20 minutes). Drain and return to the pot.
In a separate large skillet, cook the greens with butter, about 4-5 minutes until they are wilted. Add the scallions and cook another minute. Pour in the milk or cream, mix well, then add the potatoes. Reduce the heat to medium.
Use potato masher and hand mash the potatoes, mixing them up with the greens. Add salt and pepper to taste, and serve hot, with 2 tablespoons of butter in the center, which will melt.