In a medium saucepan, whisk together the cocoa powder, granulated sugar, ground cinnamon, and water until smooth.
Place the saucepan over medium heat and bring the mixture to a gentle simmer, whisking frequently until the sugar dissolves.
Slowly whisk in the milk and continue cooking for 3-5 minutes, stirring frequently, until the hot chocolate is heated through. Do not allow it to boil.
Remove the saucepan from the heat and stir in the vanilla extract.
Pour the hot chocolate into four mugs.
Top each mug with whipped cream and Christmas sprinkles before serving.