In a soup pot over medium heat, combine the cocoa powder, sugar, hot water, and salt. Stir until the sugar dissolves into a chocolate liquid.
Once it simmers, pour the milk and stir to combine it with the chocolate base. Heat it up, stirring from time to time, until the hot cocoa is hot and see steam surface. Do not let it boil; you don't want to burn the milk.
Turn off the heat, add the vanilla, and mix. You add it last to keep the flavor and prevent it from evaporating.
Ladle the hot cocoa into mugs and top them with marshmallows or whipped cream. If needed, transfer it to a crockpot to keep it warm (using the warm setting).
Notes
Maple syrup or honey can be swapped for granulated sugar (same amount). The flavor will change slightly.
Unsweetened cocoa powder is more acidic and gives a deeper, richer chocolate flavor, while Dutch-processed cocoa is milder and less intense. Both work well in this recipe.
This recipe yields a little more than 16 cups of liquid, which, when served, is about 12 mugs.