In a small bowl, whisk together the cocoa powder, sweetener, and salt. Add it to a small saucepan and add the water and whisk once more.
Place the saucepan over medium heat. Once it begins to boil, reduce to low heat. Let the sauce simmer for 1-2 minutes, stirring regularly.
Remove the saucepan from the heat and whisk in the vanilla extract. Let the syrup cool completely, before transferring it to a shallow container or jar and refrigerating it for at least an hour, to thicken.
Serving size is 3 tablespoons. TO STORE: Chocolate sauce can be stored in the refrigerator, in a sealed jar or container. It will keep well for up to 2 weeks. Remove the sauce from the refrigerator 15 minutes before using it. TO FREEZE: Place leftover chocolate sauce in a shallow container and store it in the freezer for up to 6 months.