Place the chocolate in a double boiler and melt over medium heat. Whisk in 1/4 cup hot water to incorporate.
Sift the cocoa powder into a large mixing bowl. Whisk in about 2 tablespoons milk, to form a paste. Whisk in the rest of the milk, the cream, vanilla, and salt.
Whisk the milk-and-cocoa mixture into the melted chocolate, and transfer to a medium saucepan. Bring to a boil over low heat, whisking continuously.
