It's a lot better than it sounds, honest.
Whisk the chocolate milk, water, powdered sugar, cocoa, and salt together in a small saucepan on medium-high heat. Let that stuff sit until it's boiling.
Take the pan off the heat and stir in the potato flakes. Add 1 to 2 tablespoons of water if you want to change the consistency a bit.
Let your choco-tatos chill for an hour, and it's ready. If you want some extra chocolate flavor, add in some chocolate syrup.