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 the last tablespoon (containing sediment) in bowl. Boil soaking liquid until reduced to about 1/2 cup.
Preheat oven to 350 degrees F.
Wash Porcini under cold water to remove any grit and pat dry. Coarsely chop Porcini. 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.
Courtesy of Gourmet Magazine