I use this recipe here that was posted by bunnywoman on the Wilton.com discussion boards:
ALICE'S MODIFIED ROYAL
1 pound sifted powdered sugar
3 Tbsp. meringue powder
5-6 Tbsp. water
1/2 cup of butter ( ¼ cup if flooding)
Add all ingredients in your mixer bowl and mix until nice and smooth.
You can stack cookies when dry, but they will not dry rock hard like royal icing.
I add 1 tsp. vanilla to this to give it a nice flavor. Adding...