Black cocoa has little natural acidity, an impossibly black as midnight color and a rich chocolate flavor that tastes just like your favorite cream sandwich cookies (because it’s in that cookie too!) These three elements are ideal for making a very chocolaty but not bitter drink that’s scary enough for Halloween but tasty enough for any day.
Heat the milk, sugar, cocoa powder and salt in a medium saucepan over medium-low heat. Cook, whisking, until the sugar dissolves and the mixture is smooth and steaming with tiny bubbles around the edge of the pan, 8 to 10 minutes.
Add the chocolate to the pan; whisk until fully dissolved and smooth. Keep warm.
Put the mini marshmallows in a microwave-safe bowl and microwave until puffy, 10 to 20 seconds. Stir with a fork until smooth, then let cool slightly.
Pour the hot cocoa into mugs. Using the fork, lift up some of the melted marshmallow and drape it over each mug, zigzagging strands of marshmallow across the top.