Put cocoa powder, salt and sugar in a small pot. Mix well together. Stir 250 ml of milk. Bring to a boil over high heat and stir constantly*. Fill in the remaining 500 ml of milk.
*Note: homemade chocolate milk can look a little different. For example, my recipe has no emulsifiers, and therefore the drink will sometimes require stirring to prevent the chocolate from settling. But it is also less processed and therefore healthier, because who actually needs modified starch and synthetic vitamins?