Whisk cocoa powder and milk together thoroughly in a large saucepan. Heat and whisk to dissolve the cocoa. (I use medium-low heat). Add the sugar and stir/whisk to dissolve. Be sure the sugar has dissolved before continuing on, so your syrup is not grainy. Bring to a boil and boil for 3 minutes. Be sure to watch this VERY carefully. Mine has a tendency to want to boil over immediately, so I have to continuously take it on and off of the burner. Remove from heat and stir in salt and vanilla extract. Let cool completely and skim off any skin that may have formed. Pour into a clean/sterile jar with a spout and store covered in the refrigerator.