Warren Buffett’s Protégé Is Building a Mini Berkshire

Tracy Britt Cool spent a decade working for Warren Buffett. She now wants to buy the kinds of companies that might have interested the famed investor 30 or 40 years ago.

Those are businesses typically run by founders or family owners that have solid performance and competitive “moats”—a favorite term of Mr. Buffett’s—yet aren’t big enough to draw Berkshire Hathaway Inc.’s attention today.