Separate mushroom stems from caps. Slice caps and set aside. Chop stems. Melt butter in heavy large saucepan over medium-high heat. Add mushroom stems, onion and leek and sauté until tender, about 8 minutes. Mix in chicken broth, potatoes, carrot, barley and bay leaves. Cover mixture and simmer 30 minutes. Uncover soup, add mushroom caps and continue simmering until vegetables are very tender, about 25 minutes. Season soup to taste with salt and pepper and serve.