Bring the water, sugar, and corn syrup to a boil over medium heat in a 3-quart saucier. Decrease the heat to low and gradually whisk in the cocoa powder. Continue to whisk until all the cocoa has been incorporated. Cook until slightly thickened, about 5 minutes. Remove from the heat and add the vanilla and salt. Let cool for 15 minutes, then pour into squeeze bottles using a funnel. Store in the refrigerator.