I own a business in the south of France where the summers are quite hot and I experience a problem with my cookies as soon as we reach the + 80° F temperatures.
The butter then starts to melt and seep through the royal icing, creating unsighty stains, especialy on white cookies.
I've ameliorated it a bit by adding more flour to my cookies recipe and using thicker icing for flooding but I still have issues.
I thought about a way to make the cookies greaseproof with some kind of glaze or eggwash before decorating but I haven't been successfull so far.
All suggestions welcome; thanks!