Cook the potatoes in boiling water until they are tender.
Drain well, then sieve to make a smooth puree.
Add the milk, butter and half the egg and beat well.
Put into a forcing bag and pipe onto a greased baking sheet.
Brush with the rest of the beaten egg, then bake in a preheated hot oven (220 C/425 F, Gas Mark 7) for 15 minutes.