Preheat the oven to 180°C. Peel and wash the potatoes, cut into rough chunks and cook in salted water for about 25 minutes. Drain, leave to dry then press through a potato ricer while still hot. Stir in the butter.
Separate the eggs. Stir the egg yolks into the potato mixture and season with salt, pepper and nutmeg.
Wash the herbs, shake dry and chop finely. Add the pecorino cheese and the milk to the mixture.
Whisk the egg whites stiffly with 1 pinch salt, then fold into the mashed potato with the herbs. Fill the moulds you previously buttered with the soufflé mixture to within a finger’s breadth of the rim.
Stand the moulds in a shallow baking dish, add hot water to a depth of about 2 - 3 cm and bake in a preheated oven for about 30 minutes, until golden brown.
Pay attention not to open the oven during the cooking to let it raise well and serve it immediately.