Combine sugar, cocoa, salt, and 1 cup of the milk in a large saucepan or pot. Stir over medium heat until the mixture starts to boil. Cook, stirring constantly, for 1 full minute.
Add the remaining milk and heat until scalding hot (180℉ with bubbles on the sides), but do not boil.
While the milk mixture is heating, place the chocolate chips in a microwave-safe bowl and heat in the microwave for 30 seconds. Stir and heat for another 30 seconds. Stir again and set aside.
(Optional) Whip the heavy cream on medium-high speed until soft peaks form and set aside.
When the milk mixture reaches the scalding point, remove from heat and immediately whisk in the melted chocolate until smooth.
Serve immediately, topped with whipped cream or marshmallows if desired.
Notes
This recipe can be doubled and kept warm in a slow cooker for holiday gatherings. If you accidentally bring the milk to a boil, simply strain the mixture before serving to remove any cooked milk solids.