|unsweetened (alkalized) cocoa powder|
In a small saucepan, whisk together the cocoa and sugar. Add a small amount of the boiling water and whisk until all of the mixture is moistened. Whisk in the remaining boiling water.
Bring the mixture to a full rolling boil over low heat, stirring often. Remove it from the heat and allow it to cool slightly. Whisk in the vanilla and liqueur, if using, and use while still hot (or reheat