In a saucepan, melt the butter. Add the chopped porcini, button mushrooms, leeks, carrot, celery and rosemary and cook over moderate heat, stirring occasionally, until the vegetables are browned, 15 minutes. Add the chestnuts and stock and scrape up any browned bits stuck to the bottom of the pot. Add 3 cups of the strained porcini soaking liquid and season with salt and pepper. Bring the soup to a boil and simmer over moderate heat until the chestnuts are very tender, 30 minutes. Add the cream and let cool slightly. Puree in a blender in batches and keep warm.