Unlike its iconic American counterpart, the Oreo sold in China is frequently long, thin, four-layered and coated in chocolate. But both kinds of cookies have one important thing in common: They are now best sellers.

The Oreo has long been the top-selling cookie in the U.S. market. But Kraft Foods Inc. had to reinvent the Oreo to make it sell well in the world's most populous nation. While Chinese Oreo sales represent a tiny fraction of Kraft's $37.2 billion in annual revenue, the cookie's journey in China exemplifies the kind...