I get asked all the time, how to make the swirl on a cupcake "like a frosty boy icecream". It's simple really. Use the Wilton tip 1M or 4B or if you don't want grooves in your swirl use Wilton tip 1A. If you're doing mini cupcakes, then you need tip 22, 32 or 199 (for no grooves use tip 12). You can find all of these icing tips/nozzles on the kiwicakes website.
Follow this link to Wiltons website for step-by-step instructions on how to get the look. http://www.wilton.com/technique/1M-Swirl