After i frost my cookies with royal icing, they're fine that day, not too hard but stackable. The next day after being in an airtight container overnight, the frosting turns into a rock! I've read adding glycerine helps but I can't find that in stores for some reason. Help please!

Found glycerin on wiltons website! Also read you can add corn syrup or even gumpaste powder, too

I don't use straight royal icing because like you said, it gets rock hard. I use a recipe over at KarensCookies website called "meringue powder buttercream" which is basically a royal icing with a bit of shortening in it. It may take a little longer to dry, but it gets a nice crust but is still soft to the bite and is stackable. She's got videos on her site showing her making it as well as using it. Good luck.

I have found glycerin at Michaels, Joann Fabrics and Hobby Lobby by the Wilton items. It's a small bottle near the flavorings.

http://www.karenscookies.net/Cookie-Decorating-Recipes_ep_68-1.html#recipe09

Thank you so much! Should this be the same thickness as royal icing?

Some people use two consistencies, one to outline and one to flood. I use one that's in between for both for the most part. Unless there is fine details or writing, then it needs to be a bit thicker. Sorry I'm not more helpful here. When I first make mine I do add about 1/4 tsp of water at a time to get it to the consistency that I want. Good luck.

Thanks again!

That Meringue Crusting BC recipe sounds like what I had on some cookies recently and I have been trying to figure out what it was. They were the best decorated cookies I ever had. Can't wait to try that recipe! 

I also use the meringue powder buttercream recipe from Karen's cookies. Its so much better and the icing does not get rock hard. It firms up nicely for you to stack, but still soft enough to really enjoy the cookie.

I'm definitely gonna try that instead!

http://cakecentral.com/a/how-to-make-royal-icing-that-wont-dry-rock-hard

That looks good too, thanks!