Royal icing used as mortar for gingerbread houses or for decorating sugar cookies.
Take it one step further, turn it into flooding icing for that smooth, shiny finish.
Like this:
Only you’re probably more artistic than I am and yours will look much neater. But anyway. 
All you need is:
2 egg whites
3 cups powdered sugar
some water
Beat your egg whites on medium-high/high until nice and foamy. I mean, really foamy. Then gradually add in the powdered sugar.
Voila! Royal icing.
Now, you want to flood it?
Just thin it down with water. Add a teaspoon at a time to your already separated and colored royal icing. Get it to a nice runny consistency. Not ‘watery’ mind you, but thin.
To make cookies like above (only prettier) first pipe your border with the royal icing and allow that to dry and set. Then, flood the interior. Flooding is fun. You can drop colors onto the wet flood and swirl and drag and all kinds of neat stuff I’ve yet to get the hang of. hehe but it sure LOOKS neat.
Now you have the recipe, go out and make some beautiful cookies!
