In a heavy saucepan on medium whisk water and sugar together until sugar is dissolved (about 4 minutes), whisking often.
Add cocoa and salt and continue on low heat until the sauce just starts bubbling, whisking constantly.
Remove from heat and add the vanilla.
Pour into glass jars and cool in the fridge OR serve immediately.
The sauce thickens as it cools (or, see notes below if you are using this immediately)
If you would like to thicken the consistency (this is pretty thin!) into a syrup consistency, add a small bit of water to 1.5 Tablespoons of cornstarch and add it in with the cocoa and salt. It should thicken up nicely.