Around Christmastime, I ordered a Hershey's Calender, with just a few proof-of-purchases. The calendar has a recipe for each month, and for St. Patrick's Day, there were these cookies that you make with brownie mix. It seemed simple and they looked fun, so I made them and brought them into work, and gave some to my friends. Everyone liked them, but some people were not too thrilled with the minty icing, so maybe just keep it green, but don't add the extract!!
I have to type up the recipe, because I can't find it online, but here are some pictures for now: