In a small bowl, whisk together unsweetened cocoa powder, sugar, cinnamon, and kosher salt.
In your Instant Pot, add water and your cocoa powder mixture and select the Sauté feature. Keep whisking until it gets thick.
Add in the milk and heavy cream and keep mixing until it becomes creamy and is warmed throughout. Do not allow mixture to boil.
Hit "Cancel" and serve.
Notes
Storage: You will want to put any leftover Hot Chocolate in an airtight jar (like a mason jar) and keep it in the fridge for 3-5 days. You can rewarm it on the stove top on medium low until warmed throughout.