Whisk together the cocoa, sugar, water and salt in a small saucepan. Turn the stove to medium and bring the sauce to a simmer, which will take a few minutes. Once it reaches a simmer, continue to stir for an additional 3 minutes. Don't let it get to a full boil. Turn the heat off and let the sauce cool before serving. Keep refrigerated in an airtight container for 2 weeks.