In a medium saucepan, combine the cocoa powder, sugar, water, and salt over medium heat. Heat this chocolate base mixture, stirring constantly, until smooth.
Add the ground ginger and stir to combine.
Once it simmers, reduce the heat and pour the milk. Continue stirring as it heats up and steam appears. Do not let it boil; you don't want to burn the milk.
Turn off the heat, add the vanilla, and mix.
Serve:
Pour the ginger hot cocoa into mugs and top it with marshmallows or whipped cream.