McDonald's breakfast burger mashup debuts

One month after blurring the line between breakfast and other meal times, McDonald's is going one step further with a breakfast burger mashup sold at participating Chicago locations.

The "ChiTown Classic" includes Canadian bacon, an ingredient found on the breakfast menu in its Egg McMuffin and Egg White Delight sandwiches, and applewood smoked bacon. Two locations in Chicago told CNBC they began selling the burger last week. It will remain on the menu until Dec. 27, according to McDonald's spokeswoman Lisa McComb.

Although the burger includes Canadian bacon, which is currently only on the national breakfast menu, McComb said the market does not consider it a "breakfast-lunch mashup."

For now, the burger is a regional offering. It is one of two finalists in the chain's "Chicagoland Burger Build Off" competition, which drew thousands of entries.

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But on Twitter, McDonald's corporate account has been telling customers they can also create their own mashups in restaurants by adding an egg to burgers at participating locations.

Last month, McDonald's launched extended breakfast hours, one of its biggest initiatives in years.

During its latest earnings conference call, McDonald's said breakfast sales, especially those of the McMuffin, helped contributed to its better-than-forecast results. After the chain transitioned back to the sandwich's original recipe with butter instead of margarine, it saw sales jump by double digits.

Update: This story was updated to include comments from McDonald's.