Put water, sugar, cocoa powder, and salt into a saucepan. Simmer.Simmer until the sugar has completely dissolved. Stir occasionally. Heat for 30 more seconds on low heat then take off heat and cool.
Add vanilla extract.Once the syrup has cooled a bit, stir in the vanilla extract.
Pour syrup into an airtight container.The syrup will keep for up to 2 weeks in the refrigerator.
Good quality cocoa powder is necessary to make the best tasting chocolate syrup so if you're going to spend extra money on any of the ingredients, have it be that.
Chocolate syrup will be thin right after cooking but will thicken a bit after it cools. It'll still be pourable.
Recipe makes 1 cup (16 tablespoons) of chocolate syrup. Use 1-2 tablespoons of syrup per 1 cup of coffee.