Bring 4 cups whole or low-fat milk to a simmer in a saucepan over medium heat. Mix 3/4 cup sugar, 1/2 cup unsweetened cocoa powder and 3 to 4 ounces finely chopped bittersweet chocolate in a bowl, then whisk into the milk. Reduce the heat to low and simmer, stirring, until the chocolate melts and the cocoa is thick, about 2 minutes. Divide among 4 mugs and garnish each with homemade marshmallows (see below) or whipped cream.
Photographs by Marko Metzinger/Studio D