In a small freezer-safe plastic container with a lid, mix the water and chocolate mix. Whisk or stir to dissolve the powder completely.
Let stand to cool, then cover the container and place in the freezer for 4 to 5 hours, or until solid.
To serve, let the container sit at room temperature for 10 to 15 minutes, or until the hot chocolate is just beginning to melt slightly around the edges. Or, put the container in the microwave and cook on high for about 30 seconds. (It should still be rather hard and need to be scraped with a spoon.)
Dollop with whipped topping, if desired. Serve immediately.