In a small saucepan set over low-medium heat, add water, sugar, cocoa powder, salt, vanilla extract, and espresso powder. Whisk to combine so that all ingredients dissolve and are smooth. Allow mixture to come to a simmer, whisking frequently.
Once mixture begins to simmer, add Kahlua (or preferred coffee liqueur) and continue to simmer and thicken for 2-4 minutes, still whisking frequently.
Once mocha syrup is to your preferred thickness, remove from heat and pour into a heat-safe container to cool to room temperature. Store syrup in the refrigerator.