If the potatoes are not peeled and mashed while hot and if the boiling milk is not added immediately, the Duchess potato will be lumpy and gluey. If you only have egg whites they will be fine and will make a deliciously light mashed potato also.
900g (2lbs) unpeeled potatoes, preferably Golden Wonders or Kerr's Pinks
300ml (10fl ozs/1 1/4 cups) creamy milk
1-2 egg yolks or 1 whole egg and 1 egg yolk
25-50g (1-2oz/1/4-1/2 stick) butter
Scrub the potatoes well. Put them into a saucepan of cold water, add a good pinch of salt and bring to the boil. When the potatoes are about half cooked, 15 minutes approx. for 'old' potatoes, strain off two-thirds of the water, replace the lid on the saucepan, put on to a gentle heat and allow the potatoes to steam until they are cooked. Peel immediately by just pulling off the skins, so you have as little waste as possible, put through a ricer or mouli legume while hot. (If you have a large quantity, put the potatoes into the bowl of a food mixer and beat with the spade) or mash by hand.
While the potatoes are being peeled, bring about 300ml (10fl oz//1 1/4 cups) of milk to the boil. Beat the eggs into the hot mashed potatoes, and add enough boiling creamy milk to mix to a soft light consistency suitable for piping, then beat in the butter, the amount depending on how rich you like your potatoes. Taste and season with salt and freshly ground pepper.