Cook the potatoes: peel, cut into 1-inch cubes, and cover with cold salted water; bring to a boil, then simmer 12–15 minutes until a fork slides through with no resistance; drain and steam-dry in the warm pot 2–3 minutes.

Warm the milk and butter together in a small saucepan until the butter melts and swirls in (don't boil).

Add the cauliflower to the potatoes; pour the warm milk-butter over; add the salt and pepper.

Mash by hand with a potato masher until fluffy and rustic, with small visible bits of cauliflower and potato.

Fold in the chives; pile into a bowl; press a small well in the center with a spoon and set a pat of butter in it to melt down the sides. Serve hot.