Place about 10 whole porcini mushrooms in small bowl. Cover with enough boiling water just to top mushrooms. Cover and set aside 5-10 minutes or until mushrooms have reconstituted and are soft. Drain mushrooms (save mushroom water for soups or gravy), squeeze out excess water. Thinly slice mushrooms and set aside.
Place remaining porcini mushrooms in clean coffee grinder and grind to fine powder to make 2 tablespoons.
Place butter in food processor fitted with metal blade. Add 2 tablespoons porcini powder, half of sliced porcini mushrooms, and salt to butter. Blend until well mixed and butter is smooth.
Using rubber spatula, scrape butter onto sheet of plastic wrap. Roll into log shape. Place reserved porcini slices on top of butter and wrap tightly. Twist ends of plastic closed. Refrigerate 30 minutes or until just firm enough to slice.