It's been less than two weeks since Yum! Brands' (NYSE:YUM) Taco Bell rolled out its breakfast menu worldwide, but the country's largest chain of Mexican eats is ready to take another bite out of McDonald's (NYSE:MCD).

You may have seen the first ad in which Yum! Brands found several folks named Ronald McDonald and turned them into promoters for the new Taco Bell breakfast menu that features a taco-shaped waffled sandwich and a breakfast tweak of its Crunchwrap stuffed tortilla. The ad has gone viral, with more than 1 billion earned impressions on social media, according to Taco Bell.

The quick-service giant is not resting on its laurels. Taco Bell is introducing a new ad pitching its breakfast menu, and it's still aiming at McDonald's. 

The new spot singles out the Egg McMuffin, cleverly reworking the lyrics to "Old MacDonald Had a Farm" into a dig at how dated Mickey D's signature breakfast sandwich has become. Second only to the RadioShack Super Bowl spot for bringing back the awesomeness of the 1980s, the new commercial compares the Egg McMuffin to mullets, keytars, Howard the Duck, Galaga, and even Canadian rockers Loverboy.

Taco Bell obviously isn't the first company to challenge McDonald's leadership in the morning market, but no rival has been as brazen in going for the jugular of the world's largest burger chain. It's not a bad move, as McDonald's is vulnerable. After years of consistently positive comps, the Golden Arches have been posting iffy store-level sales lately. Taco Bell also clearly feels that it has a differentiated product, and it'd better hope that it's not just the novelty of the waffle taco that is attracting attention.

Taco Bell tested more conventional offerings including oatmeal and yogurt parfaits, which not so coincidentally are options at McDonald's for diners seeking healthier alternatives to its eggy sandwiches and fried hash brown patties. However, it didn't launch with them on March 27, and now we're seeing why it's tucking those products away until later. If Taco Bell is going to attack McDonald's -- not once, but now twice within a span of two weeks -- it can't be copying its rival's menu.

One can rightfully argue that McDonald's beat Taco Bell to the breakfast burrito that is part of the new morning menu, but that low-profile entry was a given. Taco Bell was going to need a breakfast burrito. However, emphasizing the more unique waffle taco and A.M. Crunchwrap options in its marketing campaign will set it apart. Yum! Brands will keep taking swings at McDonald's until it lands a punch.