These are wonderful ideas - royal icing is by far the best way to go - just be careful not to let it harden before applying. You beat me to it. That's what I'd suggest. Use straight royal icing to pipe the borders, then thin the royal with corn syrup to flood consistency to fill in. For better sheen, set the cookies under a gooseneck lamp (or heat lamp) for 10 minutes or so to set the icing surface. The icing will dry fairly quickly to let you stack/store the cookies, but will remain chewy underneath so it doesn't shatter when you eat the cookie. To avoid the two steps of piping a border then flooding, I'll sometimes just dip the face of the cookie in royal thinned with corn syrup and clean the edges with my finger. Another option if you don't mind forgoing some of the shine is rolled fondant. While the cookies are baking, roll some fondant fairly thin, and cut it with the same cutter you use for the cookie. When the cookies come out of the oven, place the rolled fondant shapes directly onto the hot cookies. The heat will melt the back of the fondant a touch and make it adhere, avoiding the need to glue it onto a cool cookie later. Gives a good working surface with little fuss. I haven't tried it, but I bet it would work with modeling chocolate too. ←
