In a medium bowl, whisk the icing sugar with 3 tbs of the boiling water, corn syrup, and vanilla.
Add boiling water by the tablespoon, stirring constantly, until you have a thick, spreadable icing, like a thick caramel sauce. [You can always thin it out with a bit more hot water as you go.]
Ice half of the flat golden BOTTOM of each cookie with the vanilla icing, allowing to cool on the counter, bottoms up, for 30 minutes or refrigerating for 15, while you make the chocolate batch. [You will likely use less than half of the vanilla mixture!]
Into the remaining vanilla icing mixture, whisk the cooled melted chocolate.
Then, whisk the cocoa into the chocolate icing, adding more boiling water by the tablespoon, as necessary, to achieve a thick, spreadable icing with the same consistency as the vanilla, ie a nice caramel sauce.
Frost the bare halves of the bottoms of each cookie with the chocolate icing.
Allow the icing to harden for an hour before serving.
Cookies will stay fresh in an air-tight container for two days or sealed in the refrigerator for about a week. You can also place the cooled and iced cookies into a freezer bag and freeze for two to three months, thawing overnight in the refrigerator and then on the counter.