In a medium saucepan, whisk together the water, sugar, corn syrup (or agave or glucose), and cocoa powder.
Bring to a boil over medium heat. Once it’s just begun to simmer and boil, remove from heat and stir in the chopped chocolate until melted.
Let the chocolate sauce sit at room temperature for a few hours before serving to thicken.
Store, tightly covered, in the refrigerator for up to 10 days. Can be rewarmed before serving, if desired.