In a small bowl, combine the sugar and cocoa powder and set aside.
In a medium saucepan, combine the cream and water. Place the pan over medium heat. Whisk the sugar and cocoa powder into the liquid and continue to heat, continuing to whisk, until it comes to a boil. Reduce the heat to medium-low and cook, stirring frequently, until the liquid has reduced by half. Pass the sauce through a fine-mesh sieve into a bowl and let cool. The sauce can be made ahead and refrigerated, covered, for up to 3 days. Reheat in the microwave before serving.
Recipe by Bakepedia at https://www.bakepedia.com/payard-chocolate-sauce-recipe/