Boil the potatoes in a large pot of salted water for about 45 minutes or until they are thoroughly tender then drain well and return to the pan and continue cooking for a minute or two to dry out any remaining moisture.
Peel the potatoes and push through a "ricer" or mash them. Transfer the potatoes to a saucepan and add the butter, minced garlic, milk and yoghurt and salt and pepper to taste and whip with a wooden spoon while reheating until the potatoes are light and fluffy.
Fold through the chives. Divide the potato between 6 small oven-proof ramekins or arrange in one large dish and dot with the butter. Grill until golden brown on the surface and serve immediately.