In a Dutch oven over medium heat, combine whole milk, whipping cream, sugar and cocoa powder. Whisk constantly until cocoa powder is combined and sugar is melted. Add chocolate chips and stir until melted.
Heat thoroughly, then reduce heat to low to keep warm. Don't ever let the mixture come to a boil. Stir occasionally so it doesn't burn.
Serve in warmed mugs with whipped cream, marshmallows and other hot chocolate toppings.