You know who is a genius? Deb of Smitten Kitchen.
Seriously, I made her recipe for brownie roll out cookies. Delish! And they came out perfectly.
You can find the cookie recipe on her blog and I made buttercream frosting with:
-1 stick butter
-1/3 lb. confectioners sugar
-1 teaspoon vanilla
Just whip it up in your mixer until smooth and spread on cooled cookies. Don't forget the sprinkles!