How to Make It
In a medium saucepan, combine the milk with the sugar and a pinch of salt and cook over moderate heat, whisking, until the sugar dissolves, about 3 minutes. Whisk in the cocoa powder until incorporated. Add the ice cream and cook over moderate heat, whisking, until smooth and hot, about 3 minutes. Ladle the chocolate into mugs and garnish with marshmallows, if desired, and flaky sea salt.