It isn't necessary to frost shortbread, but you can do so if you want to; especially if you like your shortbread on the sweeter side. You can make a simple glaze by combining 1 cup of powdered sugar, 2 tablespoons of melted butter, and 1 tablespoon of the same syrup that you used for the cookies. If the glaze is too thick, add a little water to thin it down. If it's too thin, add a little more powdered sugar.