Combine the cocoa and sugar in a saucepan over medium-low heat.
Add a dash of salt and the boiling water. Bring the mixture to a slow boil, stirring continuously.
Once boiling, turn down to a simmer and stir continually for 2 minutes.
Stir in milk and heat until very hot, but not boiling!
Remove from heat and add vanilla. Divide between 4 mugs.
Add the half & half to the mugs of cocoa to cool it to drinking temperature.
Top with whipped cream if desired!
To store. Store hot cocoa covered in the refrigerator for up to 4 days. Reheat slowly to avoid boiling.To make ahead: This recipe is best made fresh (it's so fast too), but hot cocoa can be made 2-3 days ahead of time. Reheat hot cocoa slowly on the stovetop to avoid boiling it as it will cause the half and half to separate.