Preheat the oven to 220°C and butter a 6-cup soufflé dish.
Steam the potatoes until soft and put them in a bowl with the butter and egg yolks. Mash together well, beating in enough milk to bring the mashed potato to a slightly soft, dropping consistency. Add the snipped chives and all but 1 tablespoon of the grated cheese. Season liberally with salt, pepper and freshly grated nutmeg.
Whisk the egg whites until stiff, but not dry. Fold a third of the whites into the potato mixture to loosen it, then very gently and carefully fold in the remainder. Pour the mixture into the buttered soufflé dish and sprinkle the remaining cheese over the top.
Bake in the centre of the oven for 25 minutes, or until well risen, golden brown and soft but ﬁrm to the touch. Serve immediately as a main course with a tossed salad, or as a vegetable accompaniment to other cooked dishes such as roast poultry, grilled meat or ﬁsh, casseroles or cold cooked meats.
Vegetable Soufflé: Use half kumara (sweet potato), pumpkin or parsnip and half potato, and replace the Parmesan cheese with a mild Cheddar or Edam cheese.