Whisk together the water, cocoa powder, sugar and vanilla in a large microwaveable bowl.
Heat in the microwave for 2 minutes. Remove from the microwave and whisk it again to make sure the sugar and cocoa is fully dissolved.
Microwave for another 2 minutes. (The liquid should be quite hot and almost boiling, so depending on the power of your microwave, it may be necessary to add a little more time to the cooking time.
Whisk in the 10% cream. The cream cools the drink down, so if you like your hot cocoa hot, place it back in the microwave for a minute or two more until it reaches your desired temperature.
Pour into mugs and top with marshmallows if desired.