Mix cocoa, sugar and salt in a medium sauce pan to combine. Pour about ½ cup of the milk in and stir to make a thick paste. Turn burner on medium. Slowly add remaining milk, stirring with a whisk.
The recipe says to heat until bubbles form around the edged of the pan. But I like my cocoa piping hot so I let it go a bit longer then that. Just be sure to stir well and not let the milk scald.
Remove from heat and add vanilla.
Pour into mug and top with whipped cream or marshmallows which ever you prefer.