To make the mocha syrup, combine the cocoa or cacao powder, sugar, salt, and water in a small saucepan over low heat. Heat, whisking frequently, until the sugar dissolves and the syrup is smooth and glossy. Remove from heat, whisk in the vanilla, and set aside to cool.
To make a mocha, combine milk, coffee or espresso, and mocha syrup in a glass measuring cup or mason jar with a tight-fitting lid. Whisk or shake the mixture until well incorporated and frothy.
Pour mocha over ice into a glass. Top with whipped cream and extra chocolate syrup, if desired.