We use on the one the powdered cocoa box.  It’s pretty rich with cocoa, but you can thin it down with extra milk to taste.

Boil a couple tablespoons of water, pour in the cocoa powder and dissolve it, dump in the milk, warm it up, add in vanilla extract it whatever else you want to taste.  Dump the leftovers into a pitcher in the fridge and warm up some throughout the day as necessary.