Combine the cocoa and sugar in a saucepan over medium-low heat. Add a dash of salt and the boiling water.

Bring to a slow boil, stirring continuously. Once boiling, turn down to a simmer and stir continually for 2 minutes.

Stir in 3 1/2 cups milk and heat until very hot, but not boiling. Remove from heat and add vanilla. Divide between 4 mugs.

Serve: Add another splash of milk to the mugs of cocoa to cool it to drinking temperature, if desired. Top with whipped cream.

Store leftovers covered in the refrigerator for up to 4 days. Rewarm on the stove over low heat, stirring frequently.