5 tablespoons Madeira Melt butter in heavy large pot over medium-high heat. Add shallots; sauté 1 minute. Add fresh mushrooms; sauté until tender, about 5 minutes. Add thyme and garlic; sauté until mushrooms are golden, about 8 minutes. Stir in 6 cups broth, potatoes and porcini. Bring to boil. Reduce heat, cover and simmer until potatoes are very tender, about 25 minutes. Working in batches, puree soup in blender. Return soup to pot. Mix in Madeira and enough chicken broth to thin soup to desired consistency. Season with salt and pepper. (Can be prepared 2 days ahead. Refrigerate.) Bring soup to simmer and serve. *Available at Italian markets, specialty foods stores and many supermarkets. Makes 12 servings.