In a bowl soak porcini in warm water 30 minutes, or until softened. Slowly strain soaking liquid through a fine sieve lined with a coffee filter or a double thickness of rinsed and squeezed cheesecloth into a small saucepan, being careful to leave last tablespoon (containing sediment) in bowl. Boil soaking liquid until reduced to about 1/2 cup.
Preheat oven to 350°F.
Wash porcini under cold water to remove any grit and pat dry. Chop porcini coarse. Peel potatoes and slice 1/8 inch thick. In a large (4-quart) saucepan cook porcini and garlic in butter over moderate heat, stirring, 2 minutes. Add potatoes, reduced porcini liquid, milk, cream, 4 teaspoons chives, and salt and bring liquid to boil, stirring.
Transfer mixture to a buttered 2-quart gratin dish and bake 1 hour, or until potatoes are tender. Sprinkle gratin with remaining 2 teaspoons chives.
