Looking for a fool-proof recipe for an opaque, glossy, hard-shell cookie/petit four icing--HELP!

I second the royal icing suggestion. Here in the UK you can buy royal icing sugar, so you just add water and whisk for a very long time. Royal icing seems to have gone out of favour, with fondant being the go-to icing, but for flavour and finish, royal icing is lovely.

I would look at these two recipes from Martha Stewart:

http://www.marthastewart.com/335566/petits-fours-icing

http://www.marthastewart.com/316888/royal-icing-for-sugar-cookies