4 cups milk
1/4 cup unsweetened cocoa powder
1/4 cup sugar
4 ounces bittersweet (not unsweetened) or semisweet chocolate, chopped
Pinch of salt
Bring milk, cocoa powder and sugar to simmer in heavy large saucepan over medium-high heat, whisking frequently. Add chocolate; whisk until melted and smooth. Add salt; bring to simmer, whisking constantly until frothy. Ladle into 4 mugs. Sprinkle with marshmallows.