I use a buttercream royal icing. Recipe I got from a blog that focadima recommended. I make it about the consistency of soft peanut butter for piping and then thin it to syrup for flooding. So I pipe the edges, the put the flood icing in squirt bottles and squirt it on, then use a small paint brush to work the icing into the edges. When that dries, I pipe on any additional detail work.
They were a HUGE hit yesterday. Everyone remarked how good they tasted. I thought they tasted WAY better with buttercream RI than regular RI. And I used butter flavored crisco for the cookies. Don't know if that made a difference or not. Didn't have any margarine and almost out of butter.