Scrub and peel the potatoes and cut up any large ones. Place in cold salted water and bring to the boil. Simmer for 15–20 minutes or until tender. Drain well and return to the pan for a few minutes to dry over the heat.
Press the potatoes through a sieve into a warm bowl. Add half the butter, a good seasoning of salt and pepper, the egg yolk and half the hot milk. Beat well with a wooden spoon, adding more milk as required to make a smooth creamy consistency.
Pile the potato mixture into a shallow serving dish, ‘drizzle’ over the remaining butter, melted, and put under a hot grill until crispy and brown.