In a small saucepan over medium-high heat, warm 2 cups of the stock. Add the porcini, remove from the heat and let stand for about 20 minutes. Using a slotted spoon, transfer the porcini to a cutting board and coarsely chop. Line a fine-mesh sieve with a double layer of cheesecloth. Strain the soaking liquid and set aside. In a stockpot over medium heat, melt the butter. Add the shallot and sauté until soft, about 5 minutes. Add the porcini and button mushrooms and cook, stirring, until most of the liquid has evaporated, 5 to 7 minutes. Add the Madeira and cook until the liquid has evaporated, about 2 minutes. Sprinkle with the flour and stir to coat. Stir in the mushroom soaking liquid and the remaining 4 cups stock. Reduce the heat to low and simmer, stirring occasionally, for about 30 minutes. Working in batches, puree the soup in a blender until smooth, then return the soup to the pot. Stir in the 1/2 cup cream and season with salt and pepper. Ladle the soup into warmed bowls. Garnish with fried sage leaves or drizzle with cream. Serve immediately. Serves 6 to 8. Dried porcini mushrooms lend depth of flavor to this soup, which is perfect for a cool autumn day. The mushrooms are first reconstituted, or rehydrated, in chicken stock. The soaking liquid is strained and added to the soup base to enhance the taste. Williams-Sonoma Kitchen.