Stir together in a medium heavy saucepan:
Vigorously stir in, first by tablespoons and then in a slow, steady stream:
Heat over medium heat, stirring constantly and scraping the bottom of the pan, just until bubbles appear around the sides. Remove from the heat and stir in:
Let stand, covered, for 30 minutes. Reheat until steaming, strain, and pour into mugs. If desired, top with: