Peel the potatoes, cut in half and steam until tender appox 30 - 40 minutes. Allow the potatoes to stand for a couple of minutes with the lid off to dry further.
Meanwhile chop the scallions and to a saucepan along with the milk, reserving a little for later, bring to the boil then taken off the heat and allow the scallions to infuse with the milk.
Mash the potatoes with a masher, potato ricer or my prefered method using a electric hand mixer on it's lowest setting until you have fluffy lump free mash. Add a little of the milk and give a further mix to incorporate. Add the remaining milk and scallions and stir in with a wooden spoon or spatula.
Put the butter into the pan that contained the milk along with the reserved scallions and melt over a low heat.
Stir half butter through the potatoes. Season well with salt and pepper. Dollop the potatoes into a serving dish, create some peaks. Pour the remaining butter over allowing it to pool in the hollows.