retired Pastry Chef and Owner of Zack n Boo's Kitchen pet Treats

I don't make the kind of treats that involve frosting; mine are smaller, more training-type treats. But, from my Pastry Cheffing past, I know that it is also Cream of Tartar -- in addition to the powdered sugar -- that helps harden Royal Icing (the stuff you use to glue together Gingerbread Houses, and use as the icicles coming off the roofs of Gingerbread Houses). I have no idea what the positive, negative or neutral effects of cream of tartar may have on our canine friends, but it may be something worth some research.