Super shiny glaze for cookies
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Googled and this is just one of many sites that give the reader a recipe.
If it is a metallic shine you are searching for disco dust is always beautiful IMO.
Toba Garrett's Glace icing keeps its shine yet firms up enough to stack. Gel coloring tends to work better than paste. Paste coloring becomes splotchy, I assume because it doesn't incorporate as evenly.